Terrorist mastermind Osama bin Laden must have had an "extensive support network" in Pakistan, Prime Minister David Cameron said.
Mr Cameron said the authorities in Islamabad had "searching questions" to answer about how the al Qaida chief came to be living in a comfortable mansion in the heart of the garrison city of Abbottabad, where he was killed by US special forces.
But the Prime Minister insisted that it was not in Britain's interests to start a "flaming great row" with Pakistan, which has itself suffered enormously at the hands of militant Islam and shares with the UK in the struggle against terrorism.
Pakistan's President Asif Ali Zardari issued a forthright denial that his country harboured bin Laden, and insisted: "The war on terrorism is as much Pakistan's war as it is America's."
Writing in the Washington Post, Mr Zardari said: "Some in the US press have suggested that Pakistan lacked vitality in its pursuit of terrorism, or worse yet that we were disingenuous and actually protected the terrorists we claimed to be pursuing.
"Such baseless speculation may make exciting cable news, but it doesn't reflect fact."
But doubts persist at senior levels on both sides of the Atlantic over whether elements within Pakistan's military or intelligence community - long suspected of al Qaida sympathies - helped provide refuge to bin Laden after the 9/11 attacks in 2001 made him the world's most wanted man.
White House counter-terrorism expert John Brennan said it was "inconceivable" that he did not have a support system in Pakistan.
The head of the CIA Leon Panetta confirmed that the Pakistani authorities were not told in advance of the audacious helicopter raid because of fears that the information would be leaked.
And Mr Cameron said in a statement to the House of Commons: "The fact that bin Laden was living in a large house in a populated area suggests that he must have had an extensive support network in Pakistan."