John Prescott, number two in the Labour government when Britain took part in the US-led invasion in 2003, made the remarks in a piece to be published in the Sunday Mirror newspaper.
On Wednesday, the Chilcot report returned a damning verdict on Britain’s role in the US-led war, finding it joined the conflict before all peaceful options had been exhausted and that judgements about Iraq’s capacities were “presented with a certainty that was not justified”.
It also disclosed Blair had written to then US president George W. Bush that “I will be with you, whatever” eight months before the invasion.
Prescott, now a member of the House of Lords, wrote: “I will live with the decision of going to war and its catastrophic consequences for the rest of my life.
“In 2004, the UN secretary-general Kofi Annan said that as regime change was the prime aim of the Iraq war, it was illegal.
“With great sadness and anger, I now believe him to be right.”
Blair this week voiced “sorrow, regret and apology” over mistakes made in the conflict.
But he insisted the war was right and the world was safer without toppled Iraqi dictator Saddam Hussein.
Current Labour leader Jeremy Corbyn has apologised on behalf of the party for what he called “the disastrous decision” to go to war.
Some 150,000 Iraqi people were killed in the six years after British and American troops invaded, plunging the country into chaos and creating fertile ground for jihadist groups like the Islamic State.
A total of 179 British troops also died.