John Prescott, who was deputy to Tony Blair during his entire tenure in power, said he thought the intelligence on WMD prior to the war was not "very substantial."
Prescott also told an inquiry in London into Britain's role in the conflict that the government's top lawyer had come under great pressure to say whether military action was legal and had not been "a happy bunny."
Prescott was the last person due to give public evidence to the inquiry which has already heard testimony from Blair, his successor Gordon Brown, and other senior ministers, civil servants and military officers.
However, the committee's chairman John Chilcot said on Friday he might recall witnesses to clarify "conflicts in the evidence."
The decision to go to war was the most controversial episode of Blair's 10-year premiership, provoking huge protests, divisions within his Labour Party and accusations he had deceived the public about the reasons for invasion.
Critics have long argued that Blair promised former US President George W. Bush in April 2002 that Britain would support military action to get rid of Saddam, and then exaggerated intelligence reports about WMD.
"When I kept reading (security services' reports), I kept thinking to myself, 'Is this intelligence?'" Prescott told the committee.
"Certainly what you do in intelligence is a bit of tittle tattle here and a bit more information there. I didn't have any evidence to feel that they were wrong but I just felt a little bit nervous about the conclusions on Iraq's force off what I thought seemed to be pretty limited intelligence."
Prescott said he did not know whether Blair had made any commitment to military action in 2002 but that the former prime minister had successfully persuaded Bush to seek United Nations' backing for any action.
When they failed to get an explicit UN mandate backing war, they relied on previous UN resolutions as a legal justification for action and critics have said political pressure was applied to Britain's then Attorney General Peter Goldsmith to give his support. The inquiry has heard Goldsmith initially doubted the war's legality and only concluded it would be lawful without a specific resolution a week before the invasion.
"He was not a very happy bunny," Prescott said of Goldmsith's demeanour at the time. He knew he was being pressured for a judgment. He was saying it was legal and that was enough for us (the cabinet) to accept."
Chilcot said the five-person inquiry team now planned to go Iraq and that it might recall previous witnesses as it sifted through the information given by the more than 140 witnesses it had spoken to.
Some commentators have suggested that Blair or other figures in his team should be recalled to clarify remarks.
"We may find conflicts or gaps within the evidence. If we do, we will need to consider how best to get to the bottom of what actually happened," Chilcot said.
