Howard seized on Blair's admission he did not know the claim Iraq could use weapons within 45 minutes referred to battlefield arms.

His attack came as MPs grilled defence secretary Geoff Hoon on the issue.

Hoon denied the public had been misled and argued the 45 minutes claim was not a major basis for the war.

The prime minister's official spokesman accused the media of trying to "re-write history" by exaggerating the importance of the claim in the Iraq dossier that Saddam Hussein could deploy weapons of mass destruction within 45 minutes.

Downing Street says it never claimed Iraq could fire long-range chemical or biological missiles.

CYPRUS

Hoon is now meeting the families of six Royal Military Police officers who were killed by a mob in Iraq last June.

They are likely to ask him why the men were left exposed in a dangerous place without back-up, heavy weaponry or radio communications.

The dossier row continues to raise questions about the basis on which troops were sent to war.

On Wednesday, the prime minister caused surprise with his admission he had not known the details of the 45 minute claim when he urged MPs to vote for the war in March 2003.

The day after the dossier was published, the Sun newspaper suggested Iraqi missiles could hit British forces in Cyprus.

Hoon said he only became aware of such headlines later and in any case it was very difficult to get the media to correct inaccuracies.

'DERELICTION'

Former Commons leader Robin Cook has said he knew before his pre-war resignation that the intelligence referred only to battlefield arms and it was difficult to believe that Blair had not been told too.

Tory leader Howard said it was a "grave dereliction of duty" for Blair to fail to ask such a basic question when he was sending troops to war.

Calling on the prime minister to resign, he told BBC Radio 4's World At One: "I cannot imagine a more serious failure to carry out his duty as prime minister."

The Liberal Democrats called Blair's statement "incredible" and said the information could have shifted MPs' votes.

Hoon said he had discovered the claim referred to battlefield weapons before the war when he asked one of his officials - but that was after the September 2002 dossier was published.

Asked by the MPs' committee why he had not told Blair of his discovery, he said: "Since this was not a big issue at the time, it was not a matter we discussed."

Indeed, Blair had not mentioned the claim in his eve-of-war Commons speech and it had only become controversial many months later.

Meanwhile, Blair admitted in a heated parliamentary debate that experts had not found the banned arms he used to justify war on Iraq, but US Defence Secretary Donald Rumsfeld insisted there was no definitive proof the weapons did not exist.

In Iraq, a Yemeni man was arrested on suspicion of plotting the twin suicide bombings in northern Iraq that have now claimed 105 lives, as an Islamist group with alleged links to Al-Qaeda claimed responsibility for Sunday's blasts.