Branding Blair a war criminal for backing the invasion of Iraq in 2003 with thousands of British troops, Mahathir said that he was also "a culprit".
"He is as much a war criminal as he accuses Saddam of being a war criminal," he said. "The number of people he kills or causes to be killed is very many more than the number of people that Saddam caused to be killed."
Saddam, who was executed nine days ago, was accused of killing thousands of Shias and Kurds in massacres inside Iraq, as well as starting an eight-year war with Iran that claimed hundreds of thousands of lives.
Mahathir’s comments came as he announced plans for a peace conference to create an international movement to criminalise war.
He said the conference in Malaysia next month was being held not only because of the current global situation but also to express his disgust at the cruelty of wars in general.
Survivors of the Hiroshima bombing, the Iraq war and former detainees of Guantanamo Bay are expected to share their stories at the "Expose War Crimes: Criminalise War" event to be hosted by the Perdana Foundation, his private think-tank, early next month.
Mahathir, a strident critic of the US-led occupation of Iraq, said he wanted to show the cruelty of war which he said was no longer an option as a means of resolving problems between nations.
"War should be made a crime," he said. "Those who actively promote war should be punished as criminals."