The Government hopes to push emergency legislation through Parliament in just a week to protect the use of anonymous witnesses in certain court cases, Jack Straw, the Justice Secretary, said today.

The move follows a law lords' ruling effectively banning the use of anonymous evidence which brought about the collapse last week of a £6 million murder trial and could see dozens of recent murder convictions questioned on appeal.

Mr Straw said that would bring forward a Bill next week that he hoped could complete its passage through Parliament the following week. He said that it was "essential" for the legislation to receive the Royal Assent before Parliament rises for its summer recess.

The Justice Secretary did not go into the details of the legislation, which is being hammered out by government lawyers in a desperate race against time. Opposition parties have signalled that they will support the Bill.

Experts have warned that the law lords' ruling threatened other key trials already under way and said that up to 40 people convicted of serious crimes in London alone could walk free if witnesses refused to reveal their names in any retrial.

Lawyers for two of the four men found guilty of murdering the Birmingham teenagers Charlene Ellis and Letisha Shakespeare said already that they planned to appeal. A host of other high-profile convictions in which the jury heard evidence from anonymous witnesses could now also face challenges – including those for the murders of Michael Dosumnu, Magda Pniewska, Toni Ann Byfield and Zainab Kalkoh.

The law lords ruling came as they considered the case of Iain Davis, who was convicted of shooting dead two men in a flat in Hackney, East London, on New Year's Day 2002. At his Old Bailey trial, the judge allowed seven witnesses anonymity after they expressed a fear for their lives.

In his ruling, Lord Bingham, who headed the panel of judges, said that the conviction could have been achieved without anonymity, but that anonymity had "hampered the defence" in a way which was unlawful and rendered the trial unfair.

Mr Straw said today that the Bill would set out 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”.

But he said that anonymous evidence was already "fundamental" to the successful prosecution of many cases, including violent disorder, terrorism and murder. The new legislation will ensure that already convicted criminals would not be able to appeal on the basis of the legislative gap created by the ruling from the law lords.

The Justice Secretary also promised a virtual "sunset clause" for the new law in that further legislation was planned for the next session which would replace it, allowing MPs the chance to revisit the arguments more deliberately.

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