Blair's spokesman said on Thursday that material relating to chemical, biological and nuclear weapons will be found.
He was trying to brush off criticism from some circles over the prime minister's comments on Tuesday in which he suggested only "evidence of programmes" would likely be uncovered.
Some observers detected a tactical retreat in Blair's reference to "weapons programmes" rather than just "weapons".
That has further undermined Blair's credibility and dented his popularity particularly since weapons have not been found in Iraq more than three months after US and British forces toppled the government of Saddam Hussein.
"The prime minister is also absolutely confident that we will find evidence not only of his WMD programmes, but concrete evidence of the product of those programmes as well," the spokesman said.
The heat on Blair was also turned up by Robin Cook, who resigned from Blair's cabinet in protest over the war.
Cook said on Thursday the government would have to display Iraq weapons of mass destruction to justify Britain's decision to wage war.
"The government said quite explicitly that there were weapons. They also said Saddam had rebuilt the factiories to make more chemcial weapons. To establish that that is correct, you do have to produce the weapons," Cook said.
"You cannot now say 'well there were some scientists around who might at some time have had a capacity to develop it.' That is not what (Britain's) parliament was being told in March when it voted for war," he added.
BBC's political editor Andrew Marr has added fuel to the fire by insisting "very senior sources" have ruled out the possibility of locating weapons in Iraq.
The BBC in May reported a British government dossier on Iraq in September was "hyped-up" against the wishes of intelligence chiefs by inserting the claim that Saddam could deploy chemical or biological weaposn within 45 minutes.
The British Broadcasting Corporation is now under pressure from the British defence ministry to reveal the source who alleged that.
A parliamentary committee said this week Blair's government did not mislead parliament or doctor evidence to justify war.
But the foreign affairs committee said it gave prominence to the 45-minute claim and said "the jury is still out" on the quality of intelligence used to make Blair's case.
Blair has also come under pressure to back up his insistence that Iraq tried to buy uranium from Africa.
The White House admitted earlier this week that that claim made by US President George W. Bush was based on forged documents.
Blair's spokesman on Wednesday said Britain had "different knowledge" from the US to back up its charge set out in the September dossier.
"The UN followed up with Britain to obtain additional evidence they said they had, backing allegations Iraq tried to buy uranium from Africa. It was never provided," a diplomat familiear with the International Atomic Energy Agency, IAEA told Reuters.
In March, IAEA chief Mohammed el-Baradei told the United Nations documents supporting US and British claims that Iraq had tired to purchase uranium from Niger were clear forgeries.