Foreign Secretary Jack Straw has said he did not know full details of some of the key intelligence on Iraq's illegal weapons until after the conflict.
Straw: Learned details in June
He did not realise the claim Iraq could launch WMD within 45 minutes referred to battlefield weapons, rather than long range, until June.
Tony Blair last week admitted he had not known full details of the claim.
US Defense Secretary Donald Rumsfeld said on Tuesday he could not recall hearing the 45-minute claim.
The claim was part of the UK government's September 2002 dossier on Iraqi weapons of mass destruction arms dossier published in the run-up to the March 2003 war.
It came to British intelligence from an Iraqi military source, but its use in the dossier has caused controversy.
The latest admission from Mr Straw came in a parliamentary reply to former Tory defence minister Sir John Stanley, who asked when the foreign secretary had become aware to which Iraqi weapons the 45 minute claim in the dossier applied.
Mr Straw said he first became aware of it when he saw an early draft of the September 2002 dossier.
He pointed out that the Intelligence and Security Committee (ISC) had published the relevant passage in its report in the run up to the war.
He went on that the passage stated "intelligence also indicates that chemical and biological munitions could be with military units and ready for firing within 20 to 45 minutes".
Mr Straw said the ISC also commented that the Joint Intelligence Committee "did not know precisely which munitions could be deployed from where to where and the context of the intelligence was not included in the JIC assessment".
He added: "I became aware that the intelligence behind this assessment referred to battlefield weapons in June 2003."
Last week Mr Blair's admission that he had not been aware before the war that the 45 minute claim referred only to battlefield weapons rather than long-range strategic missiles prompted Tory leader Michael Howard to call for his resignation for failing to ask "basic questions before the war".
Asked his view of the claim, Mr Rumsfeld told reporters at a Pentagon briefing: "I don't remember the statement being made, to be perfectly honest."
Commenting on Mr Straw's revelation, Tory deputy chairman David Cameron said: "I think it is an extraordinary admission.
"It means that the foreign secretary joins the prime minister in not knowing the true nature of the threat that was facing this country," he told BBC Radio 4's World at One.
"What they both seemed to think when we went into the war was that Saddam could hit British troops in Cyprus with a long range ballistic missile carrying chemical or biological weapons, and yet we know now that's not the case."
Mr Cameron questioned why the 45 minute claim featured in the dossier four times if it was "not important".
Last year, a committee of MPs said the way the claim was worded could have led to unhelpful speculation about its meaning.
Some newspapers printed banner headlines suggesting it meant British troops in Cyprus could be attacked with weapons of mass destruction within 45 minutes.
The claim became a central part of Lord Hutton's report into the death of government weapons expert Dr David Kelly.
BBC reporter Andrew Gilligan reported that Dr Kelly had cited the claim as an example of how the dossier was "sexed up" before its publication to make the case for war.
But Lord Hutton said the allegation that the government had embellished the dossier with intelligence it believed to be unreliable was "unfounded".