The 66-year-old veteran Labour MP initially struggled to get the backing of the 35 MPs he needed, but is now the bookies' favourite to succeed Ed Miliband.
The surprise frontrunner said if he wins the leadership contest he would apologise for the Labour government leading the country into war with Iraq in 2003.
He also said that Labour would never make “the same mistake again” or “flout the United Nations and international law”.
Corbyn's announcement comes hours after his camp was forced to defend comments he made in a television broadcast last year in which he appeared to compare Islamic State militants to the US military.
The Islington MP, who fiercely opposed the war at the time, said Tony Blair had taken the country into the conflict “on the basis of deception” and that the former Prime Minister's decision to support the then-US president George W Bush to invade the Middle Eastern country had cost Labour millions of voters.
This is not the first time that the leadership hopeful has challenged Blair on this issue.
Earlier this month, Corbyn said that the former Prime Minister should stand trial on charges of war crimes if the evidence suggests he broke international law over the “illegal” Iraq War.
When asked on BBC Newsnight whether Blair should stand trial on war crimes charges, Corbyn said: “If he has committed a war crime, yes. Everybody who has committed a war crime should be.”
He continued: “It was an illegal war. I am confident about that. Indeed Kofi Annan [UN secretary general at the time of the war] confirmed it was an illegal war and therefore [Tony Blair] has to explain to that. Is he going to be tried for it? I don’t know. Could he be tried for it? Possibly.”
In a statement to the Guardian on Thursday, Corbyn said: “It is past time that Labour apologised to the British people for taking them into the Iraq war on the basis of deception and to the Iraqi people for the suffering we have helped cause.
“Under our Labour, we will make this apology.”
He went on: “It has also lost Labour the votes of millions of our natural supporters, who marched and protested against the war.
“We turned our backs on them and many of them have either withheld their votes from us or felt disillusioned, unenthusiastic and unmotivated.”
The MP said suggested that future UK military interventions will become rarer if he had control of Labour.
He said that he would “never again unnecessarily put our troops under fire and our country’s standing in the world at risk”, adding that Labour would “never make the same mistake again”.
His comments are believed to effectively rule out Labour under Corbyn from supporting David Cameron’s government in a proposed House of Commons vote to expand to Syria the current UK air strikes in Iraq against Islamic State.
In a video that surfaced online, Corbyn spoke to Moscow-funded news channel, Russia Today, about the increasing dominance of IS militants in Iraq.
He was asked: “What could shift the balance of forces here? What would it take for the Iraqi forces to regain control?”
Corbyn replied: “It requires a sense of unity among people in Iraq that want to stay part of Iraq and also an acceptance and an understanding why so many people in so many of the cities in the north have been prepared to accept the Isis forces.
“Yes they are brutal, yes some of what they have done is quite appalling, likewise what the Americans did in Fallujah and other places is appalling.”
The Labour MP's spokesman clarified his comments, saying: “Jeremy Corbyn believes the violent ideology of Isis is a vicious, repugnant force that has to be stopped - where Jeremy Corbyn talks about the need for a political solution and compromise he means not with Isis but against Isis, working across the region and beyond to choke off supplies that help fund and arm them and working with neighbouring states in the region to come to common solutions.”
The news that the Labour MP will apologise for the Iraq War has sparked a host of bizarre scenarios which people think he needs to express remorse for, including England's cricket team, TV shows and Mr Blobby's chart-topping success.
Jeremy Corbyn to apologise for yesterday at the Oval #corbynapologies— Al Murray the 318 (@almurray) August 21, 2015
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