As the abuse scandal engulfs both British and American governments, Blair has used his weekly question-time in parliament to go on the attack.
"There is no evidence whatever either of systematic abuse or of ministers or anyone else refusing to act on allegations of abuse in respect of detainees in British custody," Blair said on Wednesday.
Blair also said that photographs in the British tabloid the Daily Mirror purporting to show British troops abusing Iraqi prisoners are "almost certainly fake".
He defended his government's response to a report by the International Committee of the Red Cross (ICRC) into abuse of prisoners, which was passed to British officials in February.
Blair said he had only seen the ICRC report on Monday this week, an admission the opposition Conservative Party - seeking to exploit what are perilous times for the prime minister - seized on as incredible.
Blair said the cases of prisoner abuse in the ICRC report dealing with UK soldiers were already under investigation, so officials had not passed the report onto ministers.
Britain is investigating reported
abuses by its troops in Iraq
But that answer was not enough for the Conservatives.
"The prime minister has failed to give any explanation for why he did not see this crucial report, which was presented to his special envoy to Iraq, for nearly three months," said Conservative leader Michael Howard.
A separate dossier from human rights group Amnesty International this week has added to Blair's troubles. It accused UK troops of killing Iraqi civilians, including an eight-year-old girl, when they posed no apparent threat.
The Red Cross report, which was leaked to a US newspaper, described British troops stamping on the necks of Iraqi prisoners in an incident in which one captive died, and said some Americans meted out abuse "tantamount to torture".
The US Congress is due later on Wednesday to view another set of harrowing images of American troops abusing Iraqis.
Defence Secretary Donald Rumsfeld has warned the new pictures could make the scandal even more devastating.
Allegations against UK troops pale in comparison, but the political damage to Blair is being done. A poll on Tuesday put support for Labour at 32% - a 17-year low.
Blair's traditionally anti-war Labour party is increasingly fractious, with some now pondering a post-Blair era with the finance minister, Gordon Brown, in charge.
"We were worried about Iraq but we never imagined it would be as bad as this," one Labour MP said. "It couldn't be any worse. Iraq blots out everything else."