Vow to amend anonymous evidence law

The move comes as senior police warn that scores of serious criminals could walk free from jail after a Law Lords ruling that defendants have a right to know the identity of witnesses testifying against them.

John Yates, assistant commissioner of the Metropolitan Police, described the judgment as "potentially disastrous" and predicted it would prompt a series of appeals by dangerous convicts.

Lawyers for two of the four men found guilty of murdering Birmingham teenagers Charlene Ellis and Letisha Shakespeare have already indicated they plan to appeal.

Mr Yates told the Daily Telegraph: "This is potentially disastrous. A lot of good work being undone, and this will play out so badly in terms of those we are trying to reach out to in communities. Special measures are only used in the most extreme cases, which means these are our most dangerous criminals, people who have been jailed for up to 40 years. And they could be walking free."

But Mr Straw has indicated he could put anonymous evidence on a statutory footing before December's Queen's Speech.

"I'm looking at this as we speak," he told BBC Radio 4's Today programme. "Ever since this judgment on Wednesday this has been my major preoccupation, to ensure that we are able to change the law as quickly as possible."

The Government has already announced plans for a Bill introducing a statutory base for anonymous evidence - where witness intimidation is an issue - during the next session of Parliament. But Mr Straw said that would "almost certainly" be too long, adding: "I'm looking at this very urgently indeed."

Mr Straw insisted that the Government was committed to ensuring there was a balance between the need for a fair trial and to ensure that defendants did not go free because of a culture of intimidation. "It's absolutely fundamental that defendants should be able literally to see and hear the evidence before them," he said.

"But you then have to balance that with what actually happens in real life these days ... there is just a high level of fear which means that police can't get witness statements at an early stage from anybody who saw (the crime)."