Tony Blair says he "does not know" if he got it wrong on Iraq's weapons of mass destruction.
US troops have been searching for WMD in Iraq
He said no one could be definitive at the moment, but insisted he had been right to act on intelligence about WMD.
Mr Blair told the BBC's Breakfast with Frost he still believed that weapons of mass destruction would be found.
But asked if he now thought he had been wrong about Iraq's WMD, he said: "You can't say that at this point in time... I do not know is the answer."
The prime minister pointed out it took six months to find Saddam Hussein despite knowing that he was within the Tikrit area.
"In a land mass twice the size of the UK it may well not be surprising you don't find where this stuff is hidden," he told Frost.
Mr Blair said the Iraq Survey Group had found "a whole raft of evidence about clandestine operations that should have been disclosed to the United Nations, a network".
He said that while you could not be "definitive at the moment... you are entitled to ask what was the point of having all these elaborate concealment mechanisms if there was nothing to conceal".
The prime minister's admission that he "did not know" if he got it wrong about WMD was seen as a climbdown by the Conservatives.
Shadow Foreign Secretary Michael Ancram said Mr Blair was "hedging his bets".
"The prime minister only in July was telling us this survey group - in his mind he had no doubt - would find weapons of mass destruction," he told Channel 4 News.
"Now he is saying he doesn't know whether they will."
He said if the WMD claims were false it "raises very serious questions".
Labour minister Glenda Jackson, an outspoken critic of the prime minister, told Channel 4 News it appeared Mr Blair was "no longer believing his own arguments".