London: Some of Britain's most dangerous criminals could be freed after a court ruled that defendants should know the identity of witnesses testifying against them, a senior policeman said on Saturday.

Writing in the Daily Telegraph, Metropolitan Police Assistant Commissioner John Yates said he had "grave concerns" that criminals could walk free because witnesses would be too scared to give evidence.

A recent ruling in the House of Lords, Britain's highest court, could lead to a series of appeals by people who have been convicted on anonymous evidence, he said.

Yates said the government must draft emergency legislation after the law lords said common law gave defendants in criminal trials the right to confront their accusers and challenge their evidence.

"As a police officer rather than a lawyer, it seems perverse to me that the greater the fear and intimidation generated, the less likely there will be any chance of a successful prosecution," Yates wrote.

Wider public interest

"Of course, the common-law principle to which the law lords refer is exceptionally important; but there is also a wider public interest - that communities should not have to suffer the ravages of gun crime."

His comments came after judges quashed the murder conviction of Iain Davis, who was found guilty of shooting dead two men in east London in 2002.

Appeal judges ruled that it was unfair that witnesses at his trial were allowed to give evidence anonymously because they feared for their lives.

Yates said he worried that without evidence from such witnesses, many prosecution cases will fall apart.

It was vital witnesses be offered measures such as voice distortion and screens in court, he added.