Robin Cook and Clare Short were told by MI6 in the run-up to the invasion of Iraq that Saddam Hussein did not have any weapons of mass destruction capable of posing a threat to British security.
In separate appearances before MPs, the two former Cabinet ministers revealed for the first time the source for their damaging claims that Tony Blair misled Parliament and the public over the war.
They confirmed that they had each received regular personal briefings from officers of MI6, the Secret Intelligence Service, who had persuaded them that Saddam Hussein was not a "current and serious" threat.
Miss Short also gave MPs a graphic account of the way Mr Blair bypassed the Cabinet, tried to prevent his ministers from receiving MI6 briefings and left the decisions on the invasion of Iraq to a small, unelected "entourage" in his private office.
Mr Blair is being investigated by two separate Commons inquiries over allegations that he exaggerated the danger posed by Iraq's attempts to develop weapons of mass destruction in order to win support for the war.
Mr Cook, who resigned in March days before war broke out, said his claim that Iraq did not possess "a credible device capable of being delivered against a strategic target" had reflected "almost word for word" a briefing he received from MI6.