Legislation to stop rape complainants from being unfairly questioned about their sexual history in court is not working, a report has concluded.
Conviction rates in rape trials have been falling
The changes introduced in 2000 meant evidence of past behaviour could not be put before a jury in England and Wales unless relevant to the case.
The Home Office study said the rules were frequently "ignored or avoided".
It said there had been "no discernible effect" on reducing the number of failed prosecutions.
Data for 2004 shows the proportion of rape allegations that result in a conviction continues to fall, standing at 5.6% out of 14,000 reported offences.
The report found there was no evidence the new rules were being "widely observed".
Applications to admit evidence of sexual history were taking place during trials and being presented verbally, rather than in writing as they should be.
The report said there was a "surprising lack of knowledge" about the correct procedure to follow among nearly half the 17 judges interviewed.
Ignorance of the new rules was also thought to be widespread among barristers, it noted.
Some defence barristers were said to be timing their requests "just before or during cross-examination to create the most pressure on the complainant".
In one example a defence barrister asked a defendant if there had been blood on the bedsheets after he first had sex with a woman who made a rape complaint against him.
"This was clearly intended to suggest that C was not a virgin when she first had sex with D," the report said.
It illustrated a tactic deployed by defence teams to "evade the legislation", it added.
A spokesman for the Bar Council said: "There will be some cases where the defence will argue that it is relevant to mention the previous sexual history. This is made in writing to a judge in advance.
"We cannot accept this historical myth of a defence barrister causing stress to a defendant. It would not be in the interests of his client to be intimidating and unpleasant in front of a jury."
'Tighten up rules'
Solicitor General Mike O'Brien said he had written to the Criminal Law Rules Procedure Committee asking them to tighten up the rules.
He said defence lawyers should have written permission to question a victim's sexual history but that this was sometimes ignored.
"Some judges are allowing late application or questions raised during the trial ... and faced with a defendant risking a long prison term, decides that in the interests of a fair trial he should allow previous history to be raised," he said.
A Home Office spokesman said: "Clearly, it is not working as well as we would hope and we are taking this information on board.
"We are going to ensure that there is a vigorous and strategic approach to considering further reforms which may be necessary."
The report's three authors made a number of recommendations to tighten the legislation and ensure that it is implemented consistently.
A recent government consultation paper suggested a number of ways to boost conviction rates.
It said video interviews made with rape victims at the time of their initial complaint could be shown in court and suggested expert witnesses be called to testify about the trauma suffered.