A global campaign is to combat an expected explosion in cancer cases around the world.

The World Health Organisation (WHO) says it anticipates an extra 20m new cases a year by the year 2020, with 70% of all cases being in the developing world.

The rise is due to the ageing of the world's population.

The WHO, which opens the campaign at an international conference in London on Monday, says action is needed now to ensure there is less inequality of treatment throughout the world.

The biggest explosion of cancer cases will be in developing countries, despite the fact that they have less than 5% of the resources to treat the disease.

Although the WHO acknowledges that new treatments will mean an increase in cure rates for the disease, it says governments will have to set priorities within budget limits.

Cancer is set to explode due to the ageing of world's population

It is calling on governments to negotiate with private firms to set up national cancer programmes with clear priorities for care.

The WHO's cancer programme aims to act as a catalyst for the national programmes.

Professor Karol Sikora, of the WHO, said the programme aims to "considerably reduce" cancer at a low cost.

He believes action now could prevent five million cancer cases by 2020 and cut the death toll from 10m to 6m.

"We could potentially prevent a quarter of all cancers simply by applying existing knowledge," he said.

He added that a third of current cases are curable with modern technology and this could rise to a half in the next 25 years if action is taken now.

Sir Kenneth Calman: Priorities must be set by governments

Sir Kenneth Calman - the UK's former Chief Medical Officer who is now chairman of the WHO Executive Board - said: "The way forward is through the development of National Cancer Programmes that establish priorities for development in each country.

"These will take into account both the types of cancer that are prevalent in a country and its economy."

The WHO says priorities could include tobacco control, infection control and promotion of healthy eating. They could also ensure the availability of basic cancer surgery, radiotherapy and chemotherapy as well as specialist nurse education.