The Home Secretary will face fierce criticism on the issue next week from the Liberal Democrats, who hope to force through a measure to make the police complaints procedure independent.
The controversy follows a number of high-profile cases in which an inquest jury, or the High Court, found a detainee had been unlawfully killed but no charges or disciplinary action were brought.
Mr Straw has broken an established practice by not publishing a report from the European Committee on the Prevention of Torture, which reviewed the cases. It is believed to have called for changes to the police complaints procedure.
The committee, which oversees the European anti- torture convention and which visited Britain in September 1997, says nearly all its reports are published, by the government concerned. The Conservatives published every report that concerned them.
The evidence heard by the committee, which celebrated its 10th anniversary yesterday, centred on three cases. The first was that of Richard O'Brien, who died after being arrested for being drunk and disorderly in 1994. An inquest found he was unlawfully killed, but the Crown Prosecution Service (CPS) twice declined to act against three Metropolitan policemen. A case was brought this year after Mr O'Brien's family went to judicial review. The officers were acquitted.
In the second case, also in 1994, Shiji Lapite, a Nigerian aged 34 seeking asylum, died after a struggle with police in which his larynx was crushed and his head battered. An inquest jury found he had been unlawfully killed. There was no trial even after an inquiry into why the CPS did not prosecute.
The committee also heard about the case of Derek Treadaway, who successfully sued the West Midlands Police for pounds 50,000 after being forced to confess to robbery by having a plastic bag placed over his head. He was jailed for 15 years in 1982 and even when his conviction was quashed in 1996 no charges were brought.
Dame Barbara Mills, who was Director of Public Prosecutions, set up an inquiry into the CPS role in the case, but it concluded last year that there was not enough evidence to prosecute. Weeks later, Mr Treadaway was found dead in Amsterdam.
Simon Hughes, Liberal Democrat home affairs spokes-man, said his party would try to amend a forthcoming CPS Bill to make the complaints procedure independent.
A Home Office spokesman said the report had not been published because the Metropolitan Police had objected to the publication of some of its evidence. "Clearly we are aware of an issue between the committee and the Met and we are acting as an honest broker between the two. We have every reason to believe it will be resolved soon."
He said that rather than hold a public inquiry the department preferred to continue with efforts to cut deaths in custody by better training, safer facilities, more monitoring and better understanding of drug and alcohol problems.Reuse content