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 policemen 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 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.

"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 should be offered measures such as voice distortion and screens in court, he added.

"Many of the most challenging and notorious cases of recent years have relied in part on these measures and have helped secure heavy prison sentences for some of our most dangerous criminals," Yates wrote.

The commissioner said he feared a return to the high levels of gun crime seen in London in the 1990s.

"Shootings were a daily hazard," he said. "There was a permanent climate of fear that prevented witnesses coming forward."

The government has said tackling gun crime is one of its top priorities after a spate of shootings across the country, some involving teenagers.

Justice Secretary Jack Straw said he was considering whether to give witnesses more protection through new legislation, rather than relying on common law.

"I am looking at this very urgently indeed," he told BBC radio. "It's absolutely fundamental that defendants should be able to see and hear the evidence before them.

"You then have to balance that against what actually happens in real life when you've got very serious gun and drug crime."