Matt McGrath
Environment correspondent

There's been a rare summer warning from Britain's electricity grid operator as the heatwave puts more pressure on our power system.

The National Energy System Operator (NESO) has put out what's called an "Electricity Margin Notice" for this evening, which means it is asking those generating electricity to make extra capacity available.

Such notices are normally seen in the winter, when our demand for electricity ramps up as we warm our homes on those cold, dark evenings.

Seeing one during a heatwave is unusual.

The pressure comes from both the supply and demand sides during the hot weather.

The warm, still conditions tend to suppress wind output, while high temperatures reduce the efficiency of solar panels and conventional power plants.

At the same time, demand can rise due to the extra electricity needed for air conditioning.

NESO stresses that this does not mean that blackouts are likely, or that there is too little power to keep the lights on, and it will review the situation through the day.

Sarah Keith-Lucas
BBC Weather

Meteorologists here in the Met Office operations centre are very busy analysing huge quantities of weather data and observing closely how conditions are developing across the UK.

The forecasters here tell me the forecast is going as expected today, with temperatures rising quickly and humidity staying very high.

The next weather hazard they are looking into is the potential for some severe thunderstorms with large hail and the possibly onset of gale force gusts of wind for south-west England during Thursday night - a yellow warning has just been issued.

Image source, Met Office

The Met Office has issued a yellow warning for thunderstorms in south-west England on Thursday.

It will come into force between 18:00 and 23:59 BST, covering all of Cornwall and much of Devon.

Image source, House of Commons

Over in the House of Commons, the heatwave has just come up during this week's Prime Minister's Questions.

Responding to Liberal Democrat leader Ed Davey, Keir Starmer says the changing climate is "one of the most significant challenges of our time".

The prime minister says it used to be "common ground" that climate change was a "generational challenge", but that it is a "shame that in order to chase Reform votes" the Conservatives have "changed their minds".

"I will always maintain that we must be global leaders on climate change and we always will be," Starmer adds.

Hazel Shearing
Education correspondent

There's no set temperature at which schools are told they need to close in a heatwave - that decision is made on a case-by-case basis.

I asked Tim Warneford, whose consultancy helps schools obtain funding for their buildings, whether he expected quite so many to do so.

"I think it was a ticking time bomb," he replies, since England's ageing school estate was "not built to withstand the temperatures that we are now facing and are likely to face into the future".

"New buildings, we hope, will be designed sufficiently well that will reduce that risk, but the overwhelming number of our stock is going to require retrofit - and therefore that's going to require a certain amount of money," he adds.

A big problem is that many schools built in the 1950s and 1960s have "almost floor-to-ceiling windows" which trap in heat and are challenging to ventilate.

He adds that only schools built in the past decade or so are likely to have been designed with air conditioning units - and warns that some of those may not even be up and running because they're expensive to maintain.

Chris Fawkes
BBC Weather

Last night was the warmest on record for some UK sites, including at Brize Norton (Oxfordshire) where temperatures only fell to 22.3C - what is known as a tropical night.

From this warm and humid start, temperatures are rocketing upwards. Even before midday we've reached 31.6C at Thorney Island (Hants).

The June record of 35.6C is going to get smashed later this afternoon, with highs of 37 or 38C likely around 15:00 BST in southern England.

This would put us well inside the top ten hottest days ever recorded in the UK.

Global heating is to blame for such unprecedented heatwaves. You may remember that last month the May record was pulverised by well over 2C - it's clear that our climate has changed.

Matt McGrath
Environment correspondent

Image source, Justin Tallis/AFP/Getty Images

A meeting in London on how to adapt to extreme heat has fallen victim to the very thing it was seeking to tackle.

The session, scheduled at the London School of Economics during London Climate Action Week, was due to explore how countries and cities are responding to rising temperatures around the world.

But organisers cancelled the event after the Met Office issued a red extreme heat warning.

Chris Anderson from Practical Action, who was due to take part in the event, says: “We're fully in favour of the decision, for the wellbeing of attendees and panellists.

“But it shows that extreme weather is becoming unpredictable and moving faster than people can adapt, even in the richest countries.”

Beth Rose
Disability affairs reporter

Image source, Getty Images

As temperatures rise, the effectiveness of medications can change.

The Medicines and Healthcare products Regulatory Agency (MHRA) says heat damage can break down active ingredients in medicine.

That can stop or alter how much the body absorbs when it comes to tablets, inhalers, hormone patches, insulin, blood glucose monitors and EpiPens.

Dr Alison Cave, MHRA’s chief safety officer, says some medicines can also make you more likely to "burn in the sun, feel dizzy, or get dehydrated", especially if you’re taking diuretics or have a condition like asthma, heart disease or diabetes.

She says most medicines need to be stored at temperatures below 25C - but cars, trains and even bags and pockets could get warm enough to cause problems and keeping them in a cool bag could be useful.

She adds: "If your medicine has changed colour, developed an unusual smell, changed texture, or looks different in any way, this could signal heat damage. Ask your pharmacist for advice."

Does the warm weather have an impact on the economy?published at 11:53 BST

11:53 BST

Dharshini David
Deputy economics editor

As some trains grind to a halt - is the economy in danger of doing the same?

The answer is no - after all, whatever the disruption, most of us are still working.

And while events are being cancelled, that activity may simply be displaced if they are rescheduled. Equally, other forms of spending - on fans, ice cream and extra childcare if you’re dealing with school closures - may rise.

So, however tempting, it’s impossible to put a reliable number on this.

In the grand scheme of things, short-lived weather events don’t tend to shift the dial much in terms of overall GDP. But economic activity may be a bit sluggish overall for a few days as the temperature rises.

Miho Tanaka
Senior climate journalist

Image source, Gwyndaf Hughes/BBC

The Engine Room in Tottenham Hale normally serves as a community centre for local residents to get together for walks, and arts and crafts.

But this week it is serving another purpose - a “cool space”.

One of dozens designated across the capital offering respite from heat for residents, mostly elderly people.

The spaces have air conditioning systems, seating, free drinking water and bathrooms.

Frederica Roberts, tells the BBC the centre offers them ‘”absolute tranquillity” away from the “hustle and bustle”.

Her friend, Marion Wilson, jokes that spending time at the centre offers them “a relief especially with half of us that are menopausal. If you have that and you have heat, it's not good to be around us”.

Rory Bibb
Reporting from London

Johnny is a construction worker on a site in London where temperatures are already soaring.

He says: "It’s horrible. You’re outside all day and there’s no let up. We might be going home early, but there’s work to be done."

"We're all hot, and we're all in the same boat. No one's dropped down, touch wood,” he adds.

UK employers are issued health and safety advice on how to protect outdoor workers in these sorts of conditions.

But there is no legal maximum temperature at which work has to stop as there is elsewhere, meaning it's business as usual for people in industries like construction and farming.

Another amber weather warning has just been issued by the Met Office.

It will come into effect between 00:00 and 21:00 on Saturday, covering parts of southern and eastern England.

Image source, Met Office

Image source, Getty Images

Europe's ongoing heatwave is expected to continue on Wednesday.

France is experiencing another day of exceptionally hot weather, with more than half the country remaining under a red heat alert on Wednesday.

It comes after the country experienced its hottest June day since records began on Tuesday, with an average temperature of 29.8C (85.54F) recorded.

Temperatures are expected to peak in the Netherlands and Belgium on Friday, while Germany is expected to see the mercury rise to 40C (104F) over the weekend.

Image source, BBC Weather

Ben Rich
Lead weather presenter

The Met Office has been issuing extreme heat warnings since 2021 – and this is only the second time that a red warning that has been issued since then.

The first was in July 2022, when the UK set its new all-time temperature record of 40.3C.

When the Met Office issues weather warnings it takes two things into account: how likely it is that severe weather will happen, and what impact that weather is likely to have.

A red warning is issued when there is a high likelihood that severe weather will have a high impact, and that is definitely the case with this heatwave.

There is a risk of health impacts across the population and a threat of significant disruption to power networks and transport services.

Image source, EPA/Shutterstock

The National Rail website is warning passengers not to travel to, from, or within the red weather warning zone unless absolutely necessary.

The Met Office has previously warned that rail journeys are among the daily activities that could be impacted by the extreme heat.

A number of rail operators have altered their services, in response to the heat, alongside warnings that there could also be short-notice changes or alterations.

The operators listed on the National Rail website as having disruptions, ticket changes or travel advice are:

• Avanti West Coast
• c2c
• Chiltern Railways
• CrossCountry
• East Midlands Railway
• Elizabeth line
• Gatwick Express
• Great Northern
• Greater Anglia
• Great Western Railway
• Heathrow Express
• London Northwestern Railway
• London Overground
• LNER
• Merseyrail
• Northern
• Southeastern
• Southern
• South Western Railway
• Stansted Express
• Thameslink
• TransPennine Express
• Transport for Wales
• West Midlands Railway

Grand Central, Hull Trains, Lumo and ScotRail are all operating on a normal timetable with no planned alternations, but passengers have been advised to check before they travel in case of short notice alterations.

Sadaf Maruf
Your Voice news editor

Image source, Supplied

Helen and her husband Tony live in the northeast part of Shetland, in the hamlet of Lunna.

She got in touch with BBC Your Voice on Wednesday to say that for them it was a balmy 14C.

She says there is a lot of mention on the news of how hot it is south, "but the forecast here is for a maximum of 15C".

Helen who works from home in the pharmaceutical industry says, "if it got to 20C the sheep would probably lie around panting". The couple have lived in the hamlet of Lunna for 25 years.

She says: "I’ve put on a woolly dress and a vest on today! Even though its baking hot in the south, we prefer it here by far. Just a bit of warmth every now and then would be nice though - for us 20C is tropical."

The Met Office has extended the red extreme heat warning in place by three hours until 23:59 on Thursday evening.

The warning has also been extended south across Dorset, Hampshire and Sussex, the Met Office say.

It has also issued another amber warning for heat for Friday, covering eastern and central England, alongside the existing red warning.

Image source, Met Office

Matt Taylor
BBC Weather

Jersey experienced its hottest June day on record on Tuesday.

The Jersey Met Service reported a high of 37.2C (99F) - smashing the previous record of 33.4C, which was only set last year, by a huge margin.

It was also their second warmest day on record.

Image source, BBC Weather Watchers / Hedi

It was a notably warm start this morning, although it hasn't deterred some from getting out and about.

Here are some pictures taken by BBC Weather Watchers from across the UK.

Image source, BBC Weather Watchers / Hayley85

Image source, BBC Weather Watchers / Steve

Image source, BBC Weather Watchers / Philbert Nut

Image source, BBC Weather Watchers / Kryzstof