Veteran left-winger Jeremy Corbyn will campaign on an anti-austerity platform in the Labour leadership contest after a last-minute surge saw him secure the support needed for a place on the ballot.
Shadow health secretary Andy Burnham, shadow home secretary Yvette Cooper and shadow health minister Liz Kendall were already set for a place on the ballot paper, having already secured more than the minimum 35 formal endorsements from MPs.
Mr Corbyn reached the threshold to be nominated with just two minutes to spare before the noon deadline.
Mr Burnham received 68 nominations from MPs, Ms Cooper 59, Ms Kendall 41 and Mr Corbyn 36 and the four will battle it out in a series of public and televised hustings to replace Ed Miliband, who quit as leader after Labour's election defeat.
Islington North MP Mr Corbyn said his place on the ballot paper marks the launch of a "broader anti-austerity movement" in the country.
He said: "We secured these nominations as a result of a massive campaign across the country by Labour supporters urging Labour MPs to allow for a wide-ranging democratic debate within our party.
"Social media played a large part of this campaign.
"My candidacy marks the launch of a broader anti-austerity movement to shift the terms of political debate in this country by presenting an alternative to the socially devastating and widely discredited austerity agenda."
Some Labour MPs said they nominated Mr Corbyn despite disagreeing with his politics in order to ensure that the party's members had the widest possible choice of candidates to consider.
But Labour backbencher John Mann was scathing about Mr Corbyn's presence on the ballot paper.
"So to demonstrate our desire never to win again, Islington's Jeremy Corbyn is now a Labour leadership candidate," Mr Mann said on Twitter.
"Quite a number of Corbyn supporters saying to me that principled opposition is better than seeking an electoral majority. The elite speak."
Mr Burnham, the bookmakers' favourite for the contest, said: "I am grateful for the support I've received from colleagues across the Parliamentary Labour Party - MPs from all over the country with a breadth of different experiences and viewpoints.
"I know that Labour needs big changes to reach out and rebuild trust. That's why I have worked to secure the strongest mandate for change by working across the spectrum.
"It's now essential that we listen to hundreds of thousands of party members - many of them new members - as well as affiliated and registered supporters. This has to be more than a leadership election, it has to be a campaign for Labour to reach out to every corner of the country and win again.
"That's why I want as many people as possible to get involved and have their say. I want as many people as possible to be part of the change for a strong, united and changed Labour Party that will take the fight to the Tories and win."
He used a speech in Crewe to suggest Labour could introduce a university student loans-style system to help young people move around the country to take up technical apprenticeships.
Mr Burnham also indicated that he would back changes to the tuition fees system for university students.
"We will also need to look again at how we open up university to more people," he said.
Ms Cooper called for the UK to invest 3% of its GDP on science, innovation and technology, warning that the country was being left behind by international competitors.
She called for Government support to set incentives to boost private sector, universities and charitable investment in hi-tech projects.
"We need to encourage the start-ups. Invest in science, technology and innovation," she said.
"Make investment available to the entrepreneurs who are willing to take a risk. And make sure everyone has the chance to get into new and hi-tech jobs. But right now we're stalling."
In a highly personal speech, she set out her own background as a "comprehensive girl" who secured a place at Oxford University, she said: "Born in Scotland, brought up in England, we moved around when I was young.
"My family are from the coalfield communities and industrial towns of the North, like the Yorkshire constituency I now represent.
"But I grew up mainly in Alton, a small town in Hampshire, a comprehensive girl and my first job was about as rural as it gets - picking fruit for £2 an hour on the local farm and learning to drive a tractor too."
She said there should be no "no go areas" for Labour as she criticised the "narrow strategy" under Mr Miliband.
Ms Cooper added: "We won't deliver a Labour government by swallowing the Tory manifesto, Tory plans or Tory myths. In the end the Tories don't have the right values or the right answers for our country.
"Nor will we win by simply trying to splice together a shopping list of retail policies, targeted at different slices of the electorate. No 'vote Labour and win microwave'."
Ms Cooper, who is married to former shadow chancellor Ed Balls, said Labour had failed to show it had a "credible enough plan" for jobs and career opportunities and did not persuade voters it understood the big issues of the time such as the rise of nationalism, uncertainty over Europe and the threat posed by Islamic State.
She said: "In the 2015 election, I don't believe most people felt any party was facing up to those future challenges, or showing how we can earn prosperity and security in that changing world.
"Labour couldn't reassure those who felt threatened by change, nor could we convince those who wanted to be optimistic for their children that we had a credible enough plan for the jobs and opportunities they wanted."
Former Scottish Labour leader Jim Murphy has called for the leadership election to mark an end to the "self-indulgent and self-destructive" Blair-Brown infighting of the New Labour years.
Mr Corbyn insisted his campaign would not feature personal attacks on other candidates, following claims that the Burnham and Cooper camps were behind an attack on Ms Kendall by declaring the death of the "Taliban New Labour".
Asked about such comments, Mr Corbyn told BBC2's Daily Politics: "I do not believe in personal abuse of any sort."
Asked if the "Taliban" description was a phrase she would choose, Ms Cooper said: "That's not one I use."