LONDON — British spies were complicit in the mistreatment of hundreds of suspected militants by the United States and involved in dozens of cases of the illegal transfer of suspects, according to two reports by parliament.

The Intelligence and Security Committee spent several years looking at the actions of British security and intelligence agencies in relation to the handling of detainees overseas following the Sept. 11 attacks in the United States.

The committee found British intelligence officers were involved in incidents of mistreatment ranging from officers either witnessing torture first hand to passing on intelligence knowing it could be used in illegal interrogations.

"In our view the U.K. tolerated actions, and took others, that we regard as inexcusable," the Intelligence and Security Committee said. The committee said it was "beyond doubt" that British intelligence knew at an early stage that its closest security ally the United States was mistreating detainees.

The findings undermine Britain's claims to formally reject all forms of prisoner abuse and will raise fresh questions about whether the government should have taken a more independent approach from the U.S. after the Sept. 11 attacks.

Although the committee did not find any evidence that British intelligence officers directly mistreated or tortured militant suspects, they reveal new evidence about the scale of cases where British agents were aware of mistreatment.

One report said it had found 232 cases where British personnel continued to supply questions or intelligence to allies after they knew about suspected mistreatment.

In 198 cases, they received intelligence obtained from detainees who they knew or should have suspected had been mistreated, the committee said.

The committee also found 28 cases where intelligence agencies suggested, planned or agreed to rendition operations and three cases where they offered to make a financial contribution to conduct a rendition operation.