By Peter Gould BBC News Online

As a top QC, Michael Mansfield represents clients charged with the most serious crimes in the book.

He has won some celebrated cases, helping to clear the names of people who were falsely accused or wrongly convicted.

But after almost 40 years as a lawyer, he is worried about the impact of proposed changes to the criminal justice system.

The prosecution should not get a second bite at the cherry

Michael Mansfield QC

He believes some fundamental legal principles are being undermined.

The result, he warns, is that there could be more miscarriages of justice in the future.

Outspoken

Mr Mansfield speaks about the subject with a passion born from long experience in the courts.

Mr Mansfield is concerned over proposed changes

As a radical lawyer, he has taken up many unpopular causes over the years.

He defended men arrested during the miners' strike, and helped to secure the release of the Birmingham Six and Judith Ward, wrongly accused of being IRA bombers.

He is currently involved in the Bloody Sunday Inquiry, in Northern Ireland, representing the families of some of those who died.

Mr Mansfield once turned down the chance to become a judge, and is an outspoken critic of proposed changes to the criminal justice system.

He is concerned over what he sees as the continued erosion of the defendant's right to silence, and argues that the presumption of innocence is now being undermined.

Prejudice

He is also alarmed by the suggestion that a jury should be told about a defendant's previous convictions.

The size of the prison population is ludicrous...too often prison is used as a dustbin

Michael Mansfield QC

"In some cases, if there is a striking similarity between the cases, it can be admissible," he explains.

"But are we going to give juries details of previous convictions as a matter of course? I find that terribly risky, because there is a real risk of prejudice.

"It will make a difference, but it will be the wrong one, and it will probably lead to more miscarriages of justice."

A proposal to allow use of "hearsay" evidence is also gives him cause for concern.

"If people don't come to court to give evidence themselves, it is difficult to test their statements by questioning them," he says.

"This rule was introduced to protect us from gossip and evil rumour. To talk about redressing the balance between victim and defendant misses the point. You have to create a situation where they are equally protected."

Erosion

Another worry, he says, is the possibility that in future a defendant could be acquitted by one jury, only to face a second trial when "compelling new evidence" emerges.

"With this proposal to do away with double jeopardy, I think it is more a case of when it happens, rather than if," he says.

"It is being said that it will only involve a small number of cases. But I am concerned about the drip factor, and I think a large number of cases will get embraced.

"I think it deflects from the real point, which is that there should be a proper investigation in the first place. The prosecution shouldn't get a second bite at the cherry."

Mr Mansfield is also concerned by what he sees as the steady erosion of the right to trial by jury.

"More than 90% of criminal cases are now tried without a jury in magistrates' courts," he says.

Victims

Mr Mansfield argues that the management of criminal investigations should be placed in the hands of a judicial officer, to ensure that all the available evidence is considered.

"An investigation has to be impartial, which does not always happen, and that has led to some miscarriages of justice," he says.

"At present, investigations lack proper direction, expertise and resources."

Another problem, he says, is the reluctance of some people to come forward to give evidence in court - and not just women in rape cases.

Witness and victim support schemes should be improved, he says, to ensure that anyone who has to appear in court is kept in the picture. If a case is dropped, for example, they need to be told why.

Mr Mansfield says he welcomes a move by the government to review sentencing policy.

"The size of the prison population - 71,500 - is ludicrous," he says.

"Sixty per cent of jails are overcrowded, and that is intolerable. The government recognises that, but the courts have yet to get their heads around it.

"I am not saying you should not jail psychopaths, but prison is too often used as a dustbin."