Ministers last night angrily dismissed a call from Tory leader Michael Howard for Tony Blair to resign over the Government's Iraq weapons dossier.

His call for the Prime Minister to consider his position followed Mr Blair's admission that he did not know that the so-called "45 minute claim" only referred to battlefield weapons - as opposed to long-range missiles -when he asked MPs to vote for war.

Mr Howard described Mr Blair's failure to establish exactly what kind of weapons were referred to when the dossier said that some chemical and biological munitions could be deployed within 45 minutes as a "a most grave state of affairs".

"I cannot imagine a more serious dereliction of duty by a Prime Minister than failing to ask that basic question," he said.

"I have served in a Cabinet that took our country to war. There is no higher responsibility on the shoulders of any politician. It was the Prime Minister's duty to know every fact before he asked the public for their support.

"If I were Prime Minister and had failed to ask this basic question, I would seriously be considering my position."

Environment Secretary Margaret Beckett dismissed Mr Howard's attack as "nit-picking of the highest order".

"To use it to say 'Oh the Prime Minister should resign' - do you suppose Winston Churchill went round asking precisely the kind of munition they had in the Second World War and would that have been a valuable use of his time?" she told BBC Radio 4.

No 10 said last night that Mr Blair did not discover that the 45 minute point referred to battlefield weapons until after the war when the Parliamentary Intelligence and Security Committee launched its inquiry.

The Prime Minister's official spokesman stressed that it did not affect the statements elsewhere in the dossier that Saddam Hussein had long-range missiles capable of carrying chemical and biological warheads.

"The Government's belief, the Prime Minister's belief, was that Saddam had a WMD capability and that that capability was both battlefield and long-range," the spokesman said.

"The long-range issue doesn't rest on the 45 minute point."

Defence Secretary Geoff Hoon said that following the publication of the dossier he was told by officials in the Ministry of Defence that the 45 minute point referred to battlefield weapons but that he did not inform Mr Blair.