David Kay was responding on Saturday to recent suggestions by Blair that such weapons, whose alleged presence provided a key justification for invading Iraq last year, might still be found there.
"Anyone out there holding - as I gather Prime Minister Blair has recently said - the prospect that the ISG is going to unmask actual weapons of mass destruction, is really delusional," former chief weapons hunter Kay told BBC radio.
"There was a programme there. There was an intention of Saddam Hussein at some point to reconstitute it ... but there are not actual stockpiles of newly produced weapons of mass destruction," he said.
The issue of weapons of mass destruction (WMD) has dogged Blair since he told the British parliament in April 2002 that there was "no doubt at all that the development of weapons of mass destruction by Saddam Hussein poses a severe threat, not just to the region but to the wider world".
'We were wrong'
In his interview, Kay repeated his assertion that the WMD evoked by both Blair and US President George Bush in the build-up to war did not exist.
"We simply got it wrong. There were actually no weapons of mass destruction. Iraq was a dangerous country, Saddam was an evil man and we are better off without him and all of that. But we were wrong in our estimation", he said.
"The problem is the unwillingness to take the responsibility of saying a few simple words - we were wrong," he added.
On Friday, Blair questioned, on the resignation of CIA director George Tenet, said: "We know that Saddam Hussein had WMD, he used them. What we know also is that we haven't yet found them."
In January, Kay called for a fundamental analysis of how the US intelligence community had apparently been wrong in concluding that Iraq possessed WMD. Within days, both Bush and Blair had announced separate inquiries.