An interim report published today said British agents may have been unwilling to challenge abuse of prisoners because they did not want to jeopardize working relations with other countries.
And it says Britain "may have become inappropriately involved in some cases of rendition."
The inquiry deals with counter-terror activities that followed the 9/11 attacks.
The report raises the possibility that abuses including sleep deprivation and hooding were not raised.
It found 27 areas needing more investigation, including whether Britain should have taken more aggressive action to win the earlier release of UK detainees held at the US military base in Guantanamo Bay, Cuba.