New laws to protect the use of anonymous evidence in court cases will be rushed through Parliament within weeks.
Justice Secretary Jack Straw said an emergency Bill, still being drafted, would be published next week and pushed through before MPs leave Westminster for the long summer break on July 22.
The move comes after a Law Lords ruling against anonymous evidence led to the collapse of a murder trial at the Old Bailey on Tuesday.
Prime Minister Gordon Brown promised urgent action, amid fears that other high-profile cases would be damaged and appeals launched against previous convictions.
In a statement to MPs, Mr Straw said a "significant" number of ongoing cases, including violent disorder, terrorism and murder, were potentially affected.
But he assured the Commons that the legislation would ensure already convicted criminals would not be able to appeal on the basis of the "technical deficiency".
The legislation will set out details of the process for allowing witnesses to remain anonymous and require that a judge "will have to be satisfied that the need for anonymity is satisfied, that a fair trial will be possible and that it is in the interests of justice".
Shadow justice secretary Nick Herbert said the Opposition would try to be "constructive", but warned that the history of such emergency legislation was "not a happy one".
Mr Straw said urgent action was vital to prevent a "significant" number of serious cases being put at risk.
Although the new law would be rushed through, it would be revisited as part of a package of legal reforms due to be introduced in the next parliamentary session, he said.
