Writing for The Independent on Sunday, former UN chief arms inspector in Iraq Hans Blix says the two leaders are "clinging to straws" to justify the war.
Writing for the same newspaper, another former weapons inspector, Scott Ritter, said: "History will judge that Britain and the United States made the world a worse place with their war on Iraq."
Bush and Blair went to war on the pretext of finding and destroying deposed Iraq's alleged stockpiles of weapons of mass destruction.
But no such arsenals have been found to date.
Prospects of finding banned weapons faded further with Charles Duelfer, the chief US weapons hunter, reporting earlier this week that Iraq had ceased to possess chemical and biological weapons since the 1991 Gulf war.
Hans Blix has been a strong critic
of the invasion
But Duelfer said the Iraqi leader had hoped to renew his quest for weapons if punitive sanctions slapped on Iraq were lifted.
Saddam's "intent" to develop weapons in the future has now been seized on by Bush and Blair to justify the invasion.
"This is the new straw to which the governments concerned have begun to cling," Blix said in his article.
"Inspectors appointed by Bush to verify the president's assertions about banned weapons have had to acknowledge that the reality on the ground was totally different from the virtual reality that had been spun," Blix said.
A former Swedish foreign minister, who led the UN hunt for banned weapons in Iraq before the March 2003 invasion, Blix had earlier accused Blair of hyping pre-war intelligence about Iraq and denounced Bush's war as boosting terrorism and causing more suffering than Saddam's government had.
Ritter was equally unsparing in his criticism of the two leaders, arguing: "The world's two greatest democracies had undermined the legal framework of the United Nations at exactly the time when the world needed multilateralism most, to fight a global war on terror.
No banned weapons have
been found in Iraq
"History will show that it was the US and Britain that consistently operated outside the spirit and letter of international law in their approach towards dealing with Saddam," he said.
"Saddam is gone and the world is far worse for it - not because his regime posed no threat, perceived or otherwise, but because the threat to international peace and security resulting from the decisions made by Bush and Blair to invade Iraq in violation of international law make any threat emanating from an Iraq ruled by Saddam pale in comparison," he wrote.
Ritter, a former intelligence officer in the US marines who has been extremely critical of US war plans, was an inspector in Iraq for eight years.
He resigned in 1998 citing a lack of UN and US support for his tough disarmament methods.