An emergency law allowing the use of anonymous evidence, such as the testimony used to convict the killers of Birmingham teenagers Charlene Ellis and Letisha Shakespeare, in court will come into force today after being rushed through Parliament.

The new Act was drawn up after Law Lords questioned the legality of convictions based on evidence from witnesses whose names were kept secret.

Ministers moved swiftly to clarify the law in England and Wales so that witnesses can still have their identities protected if there is a risk defendants will try to intimidate them or their families.

The new Act will only have a temporary lifespan after the legal profession expressed a series of concerns about possible long-term implications, and will be replaced by other legislation later.

The House of Lords ruling last month has already led to the collapse of a £6 million Old Bailey murder trial. There were fears that hundreds more cases could be damaged, and that even convicted criminals – including murderers – could try to get court decisions overturned on the back of the ruling.

Lawyers for two of the four men found guilty of murdering teenagers Charlene Ellis and Letisha Shakespeare in Aston 2003 had said earlier this month that they planned to appeal in the wake of the Law Lords’ ruling. The case was the first in which anonymous evidence was used.

Since then, it has become an increasingly common weapon for police in trials where witnesses could face intimidation, such as cases involving gangs.

Justice Secretary Jack Straw said: “Those who intimidate or threaten witnesses to prevent them testifying must not escape justice. This Act will make sure that, where necessary, anonymous evidence can continue to be given so that we can bring the most violent and dangerous criminals to justice.”

The Criminal Evidence (Witness Anonymity) Act - which receives Royal Assent today, becoming law - includes a specific clause designed to ensure that criminals already prosecuted in cases involving unnamed witnesses cannot attempt to have convictions quashed solely because such evidence was used.

Mr Straw said: “It was imperative to legislate immediately but I am well aware of the perils of emergency legislation and for that reason there is a ‘sunset clause’ in the Act and we will be subsuming the Act into a Bill to be introduced in the next session of Parliament. This will enable Parliament to consider the experience and drafting of this Act in the interim.”