Scott Ritter said the "aggressive warfare" of the United States and Britain in Iraq was similar to German actions in Europe in the second world war.
"Both these men could be pulled up as war criminals for engaging in actions that we condemned Germany [for] in 1946," Ritter said in an address at the Chatham House think-tank in London on Friday.
"Tony Blair and George Bush are guilty of the crime of planning and committing aggressive warfare."
Ritter was a UN weapons inspector in Iraq between 1991 and 1998, when he resigned, citing a lack of UN and US support for his tough disarmament methods.
In his speech he also mocked Britain's supposed "special relationship" with the United States, likening London's position to that of a "disregarded mistress".
The sharing of information was a one-way system, with the US benefiting from UK intelligence, Ritter argued.
"Britain gets nothing, other than to say they are America's closest ally in Europe," he said.