Media playback is unsupported on your device Media caption Liam Allan had been on bail for two years before his trial collapsed

All current rape and serious sexual assault cases in England and Wales are to be reviewed "as a matter of urgency" to ensure evidence has been disclosed.

Director of Public Prosecutions Alison Saunders warned the review could see "a number of cases" dropped.

It comes after the collapse of several rape trials because evidence had not been shared with defence lawyers.

BBC legal correspondent Clive Coleman said there had been a failure to share digital evidence in each of the cases.

Attorney General Jeremy Wright said there was no evidence of "widespread malpractice or dishonesty", but police and prosecutors needed to get to grips with the way they handled electronic evidence.

In the lead up to criminal trials, police and prosecutors have a duty to disclose evidence or information that might either help the defence case, or harm the prosecution's case.

However, the system has come under scrutiny after the collapse of a number of trials, heightening concerns that evidence is not being disclosed early enough - or that rules are not being followed.

Concerns have also been raised that potentially key information taken from mobile phones, computers and social media is not being shared.

'Deep-rooted' problems

The Crown Prosecution Service, the National Police Chiefs' Council (NPCC) and the College of Policing have launched an "improvement plan" to tackle the issue.

It will include reviewing training, developing specialist disclosure experts in every police force, and providing all multimedia evidence to the defence digitally.

Ms Saunders - the most senior public prosecutor in England and Wales - said the steps would help to deal with "deep-rooted and systemic" disclosure issues which were of "great concern".

"Changes in society, such as the vastly increasing use of social media and mobile phone messaging, bring challenges that all parts of the criminal justice system, despite the resourcing challenges, must deal with," she said.

"We are taking steps to identify any individual cases of concern as a matter of urgency."

Image copyright PA Image caption Ms Saunders said the CPS was taking steps to identify cases of concern as a matter of urgency

Last week, a rape charge against Oxford University student Oliver Mears was dropped on the eve of his trial, after a diary which supported his case was uncovered.

And in December, the trial of Liam Allan, who faced 12 counts of rape and sexual assault, was dropped when it emerged evidence on a computer disc - which police had looked through - showed messages from the alleged victim pestering him for "casual sex".

Earlier this week, the BBC revealed the number of prosecutions in England and Wales that collapsed because of a failure by police or prosecutors to disclose evidence had increased by 70% in the last two years.

Social media traffic

Our correspondent said the review was "unprecedented" despite coming after years of warnings about disclosure issues by lawyers.

He said the plan revealed little about how changes would be funded, or about whether there was scope for reviewing past convictions.

It also begged the question of why the review was confined to rape and sexual assault cases when many believed problems of disclosure were systemic, he added.

On Friday, the trial of three people held on trafficking and prostitution charges was stopped after it was discovered social media evidence had not been disclosed.

Attorney General Jeremy Wright said that although he had not seen evidence of malpractice or dishonesty in the prosecution system, in some cases people were "not doing their job properly".

"But I think it also shows something else - what we're seeing over the last year or two or three, is a huge increase in the volume of particularly electronic material that features in criminal cases.

"There really is no excuse for investigators and prosecutors not getting to grips with social media traffic and text messages," he told BBC Radio 4's Today programme.

Police failures

Chief Constable Nick Ephgrave, the NPCC lead for criminal justice, said the disclosure of evidence had too often been seen as an "administrative task" finished at the end of a case, and now needed to be firmly embedded in the "investigative mindset".

He said problems had been "exacerbated by the rapid expansion of digital material involved in almost every case".

"Reviews of recent cases have shown a range of issues leading to failures but there has been no intention by officers to conceal information," he added.