Mr Cameron said a speedy judge-led inquiry was needed
Allegations that British intelligence agencies were complicit in torture should be fully investigated by a judge-led inquiry, say the Tories.
The claims by a former Guantanamo Bay detainee have been referred to the attorney general but David Cameron said that did not go far enough.
He said it would only look at whether a crime was committed not whether "our moral authority has been maintained".
The PM condemned torture and said "all the evidence" would be looked at.
Binyam Mohamed returned to the UK last month after four years in Guantanamo Bay.
He claims evidence against him was obtained through torture - and says MI5 was complicit because it fed questions to US agents.
The home secretary referred the claims to the attorney general - the government's chief legal adviser - in October 2008. She has been reviewing material about the case and is still considering whether to launch a criminal investigation.
But at prime minister's questions on Wednesday, Mr Cameron said a "brief, judge-led inquiry" was needed - whatever Baroness Scotland's findings.
"Either the attorney general will find a crime has been committed, in which case there will be a clamour for an inquiry to answer how on earth this was allowed to happen," Mr Cameron said.
"Or, on the other hand, if the attorney general decides not to go ahead with a prosecution, we still won't really have the answers to what happened in this important case."
He told the prime minister: "Isn't it also important that you are satisfied that Britain has, throughout acted with moral authority over and above the question about whether a crime has been committed?"
He said an inquiry needed to investigate what "procedures and processes are in place to ensure that Britain cannot knowingly or unknowingly be implicated in torture".
Mr Brown responded by saying the government "unreservedly condemns the use of torture".
"Under no circumstances will we participate in, encourage or condone the use of torture for any purpose," he said.
He said the attorney general, Baroness Scotland, was looking at "all the evidence" and the allegations would be examined in the courts, if necessary.
The intelligence and security committee - which oversees the work of the intelligence agencies - had already carried out inquiries into "some aspects of these matters", he said and would continue to look at them.
The committee had "looked at the issue of rendition in great detail" in a previous inquiry, he said.
He said the committee "at this stage is the best way to proceed" adding: "Let us hear the report of the attorney general."
But Edward Davey, for the Liberal Democrats, said Mr Brown's answers were "inadequate considering the severity of the allegations".
He added: "Many question whether the attorney general can be seen to be truly independent on this. Given that she has dragged her feet for many months, people are rapidly losing faith in her investigation."
Mr Mohamed, 30, an Ethiopian national who had been living in the UK since he was 15, arrived back in the UK after charges against him were dropped.
He was arrested in Pakistan in 2002 and claims he was flown to Morocco and Afghanistan before finally being taken to Guantanamo Bay in 2004.
On Tuesday in a report by UN special rapporteur Martin Scheinin, the UK was named in a list of countries accused of helping US "renditions" of terrorism suspects - either by providing intelligence or seizing suspects.
The Foreign Office said Britain condemned any "extraordinary rendition" transporting people to be tortured.