A British government inquiry into police interrogation practices in Northern Ireland has concluded that suspected IRA terrorists have been physically mistreated by Ulster police trying to extract confessions from them.

The report's conclusions, based on medical evidence and interviews with police and medical informants constitute the British government's first official acknowledgement that the Ulster police have mistreated some IRA suspects.

The government committee of inquiry, headed by Judge Harry Bennett, has found numerous cases in which the Irish-Republican Army suspects in Ulster police custody suffered serious injuries that "were not selfinflicted," according to sources with knowledge of the report's contents.

The committee reportedly recommends that interrogations of IRA suspects in Ulster be monitored by senior police officials on closed-circuit television, that the police complaint process be improved and that terrorist suspects be allowed to contact a lawyer within 48 hours of their arrest, a right they do not have under the British Prevention of Terrorism Act.

Northern Ireland Secretary Roy Mason reportedly was disturbed by the findings and has decided to carry out as many of the committee's recommendations as possible.

Mason received the report several weeks ago but had not planned to present it to Parliament or the public until next week. Highlights of the report were confirmed by informed sources here today after some had been revealed by The Guardian newspaper.

Last Sunday, an Ulster police surgeon, Dr. Robert Irwin, said on television here that he had examined many suspected IRA terrorists who were seriously injured during police questioning at the Royal Ulster Constabulary's Castlereigh interrogation center in Belfast. His statements were attacked this week by Ulster authorities including the chief constable of the Royal Ulster Constabulary, who said Irwin had made formal brutality complaints in only 10 cases.

Mason has refused comment to reporters or Parliament since Irwin's television interview created new controversy about Ulster here. In the past, Mason has strongly praised the Ulster police for helping the British Army reduce sectarian violence and killing during his 2 1/2 years as the British official in charge of Northern Ireland.

Mason has been gradually withdrawing British soldiers from Northern Ireland, turning over more and more of the responsibility for security there to the Royal Ulster Constabulary.

The Bennett Committee inquiry began last summer after Amnesty International detailed 78 cases of alleged brutality at the Castlereigh interrogation center. The committee was authorized to report on interrogation practices generally, rather than investigate individual cases in depth.

Critics of the British government's Northern Ireland policy are not expected to be satisfied with the Bennett Committee's report for this reason. Gerry Fitt, a Catholic Social Democratic and Labor Party member of Parliament from Northern Ireland, said today that general conclusions about brutality would not be enough. He said he wanted every individual case investigated thoroughly and the police responsibile for mistreatment brought to trial.

The Bennett Committee's report and Irwin's televised testimony about brutality came at particularly inopportune time for the British government, which assumed direct rule over Northern Ireland in 1972.

Next week, Parliament debates the further extension of the five-year-old "temporary provisions" of the Prevention of Terrorism Act, under which terrorist suspects can be detained without charges for seven days and can be convicted of terrorism, usually on their own confession, by a judge sitting without a jury. Opponents of the act are expected to use the new revelations of police brutality as reasons the detention powers and the reliance on confessions should be restricted.

The British government also has become increasingly concerned about the IRA's propaganda efforts abroad, particularly in the United States, to discredit a recent crackdown on violence in Northern Ireland.

The IRA has drawn attention to several hundred IRA prisoners living without clothes or blankets in cells fouled by human waste in the Maze Prison in Ulster. Northern Ireland officials have emphasized that the prisoners themselves have refused to wear clothes or use tollet facilities in protest against the British government's refusal to classify them as political prisoners.

The European Commission on Human Rights, which found Northern Ireland guilty several years ago of mistreating IRA suspects imprisoned without trials, is now investigating conditions as the Maze Prison. To present their side of the story about the prison, which was built as a model facility, Northern Ireland officials have taken members of Parliament on a tour of the prison and plan to take reporters inside soon.