Civil servants are also drawing up plans to force lawyers to reveal their defences in advance, so the prosecution cannot be "ambushed" with a surprise strategy in court, and there may be restrictions on the amount of police information that suspects can demand to see.
Dozens of cases have collapsed because the police have preferred to let drugs and robbery suspects and animal-rights "terrorists" go rather than comply with judges' orders to reveal undercover sources.
After rulings in the Guinness fraud case and the successful appeal of Judith Ward against her conviction for an IRA bombing, the defence can now get mountains of relevant and irrelevant evidence.
Earlier this year, lawyers defending Robert Black, the paedophile who lured three girls to their deaths, asked for, and received, 23 tons of police documents on the 10-year investigation.
Asil Nadir's defence team in the Polly Peck fraud case forced the prosecution to produce one million pieces of paper.
The Home Office legislation, which will be presented to Parliament within a year, seems set to cut the amount of material the defence can see.
But senior police officers' additional demand that more be done to protect the identities of their informers appears to be have been met with a refusal to act.
Although police say that judges are more willing now to protect informants than they were two years ago, major trials are still being abandoned.
In September, the case of a Turkish man and woman charged with smuggling £12m worth of heroin collapsed after the prosecution said they could not name an informant.
One officer involved in consultation with the Home Office about the scope of new legislation said that civil servants had rejected bringing in statutory rules to protect police sources, and had decided to keep decisions on whether informants should be unveiled in the hands of judges.
Ministers are worried that Lord Justice Scott's inquiry into the arms-to-Iraq scandal will heavily condemn the Government's attempt to suppress the evidence that eventually cleared the three Matrix Churchill defendants, who were charged with illegally exporting weapons-making machinery to Baghdad.
In the climate that will follow the publication of the inquiry report, it will be difficult to legislate to hide the identity of informers, they believe.
"It makes me tear my hair out when I hear the argument," said the officer. "The evidence that they tried to keep from the Matrix Churchill jury and protecting the lives of informants have nothing in common. But they are being linked."
Defence lawyers are nevertheless appalled by the success of the police in getting the Government to accept at least part of their agenda.
Anthony Edwards, a member of the Law Society's criminal committee, said: "They have been running a very clever and very political campaign to make the public believe that criminals are walking free."
Detectives respond with a dossier of cases from around the country. The bulk of evidence that defence lawyers can now collect means that suspects who five years ago might have been convicted are nowadays being set free.
One Scotland Yard officer cited the example of the prosecution of an alleged rapist in the South of England. The police had good forensic evidence that he was the rapist. But the defence was able to demand details of every rape in the area and every person the police had interviewed.
"They found that in routine house-to-house inquiries we had interviewed a man who had a rape conviction years ago," the Scotland Yard officer said.
"They put him in the witness box and asked whether he had been questioned and whether he had ever been convicted.
"The jury got the impression that he was the guilty man, even though he had nothing to do with the case."
A similar tactic was used in the Midlands by lawyers defending a man accused of robbing a supermarket. The defence obtained details of all other supermarket robberies in the area and took their client through them.
The suspect was able to prove that he had alibis for the other cases and was acquitted even though the police had not alleged that he had had anything to do with them.
Chief Superintendant Brian Ridley of Scotland Yard, who oversees prosecutions in London, said that no police force wanted to withhold evidence vital to a suspect. However, police did want legislation to prevent "fictitious arguments and accusations beingput forward by the defence".
Anne Owers, the director of Justice, the law-reform pressure group, said she viewed changes in the law of disclosure with trepidation.
The Guildford Four, Judith Ward, Stefan Kiszko (who was jailed for 16 years for a child murder he did not commit) and many other victims of miscarriages of justice were all freed after evidence that the prosecution and the police failed to disclose during the original trials was finally brought to the Court of Appeal.Reuse content