Prime Minister Tony Blair should resign because he failed to ask "basic questions" on claims made in his Iraq dossier, Michael Howard has said.
Mr Hoon answered questions from MPs
The Tory leader seized on Mr Blair's admission he did not know the claim Iraq could use weapons within 45 minutes referred to battlefield arms.
Cabinet minister Margaret Beckett branded his criticisms as "nitpicking".
"What matters is what they [the weapons] can do, not how they are delivered," she told BBC Radio 4's PM.
Mrs Beckett said wartime prime minister Winston Churchill had not wasted time trying to find out what munitions the enemy had.
Mr Blair'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.
Hoon met relatives of the military police officers killed in Iraq in June
Downing Street says it never claimed Iraq could fire long-range chemical or biological missiles within 45 minutes.
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.
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 Mr Blair had not been told too.
Tory leader Mr Howard said it was a "grave dereliction of duty" for Mr 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."
But Liberal Democrat defence spokesman Paul Keetch said Mr Howard's criticisms rang hollow because of Tory "cheer-leading" for the war.
The day after the Iraq dossier was published, the Sun newspaper suggested Iraqi missiles could hit British forces in Cyprus.
Grilled by the Commons defence committee on Thursday morning, Defence Secretary Geoff 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.
Mr Hoon said he had discovered the 45 minutes 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 why he had not told Mr Blair of his discovery, he said: "Since this was not a big issue at the time, it was not a matter we discussed."
Indeed, Mr Blair had not mentioned the claim in his eve-of-war Commons speech and it had only become controversial many months later, he added.
The Butler Committee, set up to examine the pre-war intelligence on Iraq's weapons, met for the first time on Thursday afternoon to discuss its remit.
On Thursday, the Commons defence committee questioned Mr Hoon about claims that inadequate or missing kit led to some soldiers' deaths
RMP TROOPS KILLED IN IRAQ
Corporal Simon Miller, 21
Tyne and Wear
Sergeant Simon Alexander Hamilton-Jewell, 41
Corporal Russell Aston, 30
Corporal Paul Graham Long, 24
Lance-Corporal Benjamin John McGowan Hyde, 23
Lance-Corporal Thomas Richard Keys, 20
Bala, N Wales
Mr Hoon said equipment shortages had been more widespread and serious than previously thought but the operation had been an overall success.
He later met relatives of six Royal Military Police officers who were killed by a mob in Iraq last June.
They asked him about claims the men were left exposed in a dangerous place without back-up, heavy weaponry or radio communications.
Tony Hamilton-Jewell, whose brother Simon was one of the victims, said Mr Hoon had promised to look at whether a public inquiry into the deaths could be held.
The Army has already begun its own investigation.