* British envoy says plans to release part of inquiry
* U.N. investigator talks of US-led "criminal matrix"
* U.S. delegation says still studying U.N. report
* ACLU denounces impunity for torture as "abhorrent"
By Stephanie Nebehay
GENEVA, March 5 (Reuters) - Britain will publish parts of a confidential report on its role in the U.S. "rendition" of foreign terrorism suspects, its envoy to the main U.N. human rights forum said on Tuesday.
The announcement came in response to a demand from a special U.N. investigator that the United States and Britain publish their own findings on rendition, a policy used under former U.S. President George W. Bush to snatch suspected Islamist militants abroad and interrogate them in secret detention.
Ben Emmerson, counter-terrorism investigator for the United Nations Human Rights Council, said on Tuesday it must hold "all states including the most powerful nations in the world to account".
"The exposure of the criminal matrix organised by the Bush-era CIA (the U.S. intelligence agency), from the heart of the world's most powerful democracy, now calls for an unequivocal response from all of the states that took part in the programme," Emmerson said.
The U.S. delegation said it was still studying a U.N. report issued a day before and its "substantial detail".
But Britain's Ambassador Karen Pearce told the 47-member state council that London would publish at least some of the conclusions of an inquiry by judge Peter Gibson who was asked in 2010 to examine whether British agents were involved in mistreatment or rendition of detainees held by other countries. His 2012 interim report has not been published.
"We are looking carefully at the contents of the report by the Gibson Inquiry on its preparatory work, with a view to publishing as much of it as possible," Pearce said.
The inquiry had not yet formally started due to related police investigations still under way, Pearce said.
"We fully intend to hold an independent, judge-led inquiry once these investigations are completed but we are not in a position to provide a timetable given these ongoing inquiries."
Emmerson says the "war on terror" waged by Bush after al Qaeda attacks on the United States on Sept. 11, 2001 led to "gross or systematic" violations involving secret prisons for Islamic militant suspects, clandestine transfers and torture.
Bush said in his memoirs that he had ordered the use of "waterboarding" which many rights experts consider a form of torture banned by international law.
A U.S. Senate select committee on intelligence, chaired by Democratic Senator Dianne Feinstein, investigated the CIA secret detention and interrogation programme, including the use of waterboarding, which simulates drowning. Its report was completed in December 2011 but not published.
"As the state that was in the past at least the architect of this web and an international conspiracy of crime, it seems somewhat disappointing that once an investigation has taken place that its results should currently remain secret," Emmerson said at Tuesday's debate.
Jamil Dakwar of the American Civil Liberties Union (ACLU) also called for the 6,000-page Senate report to be released.
"Definitive evidence has come to light that Bush administration officials committed serious crimes in violation of both U.S. and international law by authorising the torture and abuse of detainees in U.S. custody," he told the U.N. forum.
Although President Barack Obama's administration had disavowed torture, "it has shielded former senior government officials who authorized torture and abuse from accountability, civil liability and public scrutiny," Dakwar said.
"We unequivocally reject President Obama's statement that 'we need to look forward as opposed to looking backwards'," he said. "Impunity for torture is abhorrent and we will continue to press for accountability - both in the United States and overseas." (Editing by Robin Pomeroy)