Desperate legal moves were taking place behind the scenes tonight as ministers worked to secure the use of anonymous witnesses.
Officials were in a race against time to plug a gap in legislation ripped wide open 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, 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.�
Justice Secretary Jack Straw said the law will be changed before the December Queen’s Speech, but police chiefs said this was too long to wait.
In their judgment, the Law Lords said it has been a fundamental principle of English Law that a defendant should be able to see and challenge his accuser.
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.
Giving defendants anonymity through screens, false names and voice altering equipment has been a key weapon in convicting dangerous villains.
Police fear legal principles clash with the pragmatic demand of getting the most violent offenders off our streets.
Giving anonymity to witnesses has proved especially successful in cases of gangland violence, organised crime and terrorism.
Officials must now clear space in the Parliamentary timetable so that legislation can be pushed through before the summer recess.
Assistant Commissioner Bob Quick, head of New Scotland Yard’s Counter Terrorism Command, said action must take place as soon as possible.
He said: “This needs resolving. The Queen’s Speech is too far away, in my view.
“The implications for the fight against organised crime and terrorism are very serious.
“We urgently need this redressed, by legislation if necessary.
“It’s catastrophic. There is too much principle and not enough pragmatism in the criminal justice system.�
Assistant Commissioner John Yates said: “This is exactly the type of problem we anticipated in the wake of the Law Lords ruling.
“This was one of the most serious cases involving special measures for witnesses and
demonstrates the implications for other cases, both from the past and in the future.
“We are working very closely with all parties and are receiving active support from Government in seeking a solution.�
Ken Jones, president of the Association of Chief Police Officers (Acpo), said anonymity had become a vital tool in difficult prosecutions.
He said: “Anonymity has been used in a tiny, tiny minority of cases and the fundamental principle that you are entitled to hear from your accuser has not been breached.
“These powers are used only in rare and exceptional circumstances - the judiciary have supported them for a long time and there are rigorous checks and balances.�
The Crown Prosecution Service (CPS) said the number of cases which would be affected by the ruling was not known.
A spokeswoman said: “We have asked our prosecutors to seek an adjournment on all cases using anonymous witnesses to allow us to assess the implications of the House of Lords’ judgment in each particular case.�