Legal moves are taking place behind the scenes as ministers work to secure the use of anonymous witnesses.

Officials are trying to plug a gap in legislation opened up by a controversial House of Lords ruling last week.

Top police officers and prosecutors want the use of anonymous witnesses to be enshrined in law before Parliament rises for the summer recess.

If a solution cannot be found in the next 28 days, the courts could face a deluge of appeals at a potentially huge cost to the taxpayer.

Police believe up to 40 people convicted of serious crimes in the capital alone could walk free if witnesses refuse to reveal their names in a retrial.

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

A host of other high-profile convictions in which the jury heard evidence from anonymous witnesses could now also face challenges.

They include the murders of Michael Dosumnu, Magda Pniewska, Toni Ann Byfield and Zainab Kalkoh.

Meanwhile dozens of trials across England and Wales were effectively put on hold as prosecutors assess the wide-ranging implications of the ruling.

Among the cases thought to be affected are some of the most high-profile gang-linked murders of recent years.

Speaking at a meeting of senior officers in Liverpool, Home Secretary Jacqui Smith said she shared the frustration of the police.

But she refused to confirm if an emergency bill would be necessary to rescue the use of anonymous witness.

She said: "This is something we are looking at very urgently including, if necessary, looking at whether we can change the law.

"I certainly accept, and I said some time ago, that there is a problem here that we need to solve."

The implications of the ruling became clear at the Old Bailey today when a £6 million trial of two men accused of murder collapsed.

Judge David Paget told an astonished jury that the two-month case had been "derailed" by the decision.

He told them: "You have heard evidence from a number of witnesses that you should not have heard."

Douglas Johnson, 27, and David Austin, 41, will be retried over the contract killing of east London businessman Charles Butler, 50, next year.

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