Labour leadership: Jeremy Corbyn completes the line-up
Four candidates will compete to become Labour's new leader, after left-winger Jeremy Corbyn secured enough nominations to get on the ballot.
Mr Corbyn reached the 35 MP threshold just two minutes before the noon deadline, helped by colleagues wanting to widen the range of candidates.
He joins Andy Burnham, Yvette Cooper and Liz Kendall in the contest.
Ms Kendall's campaign team has complained to Mr Burnham and Ms Cooper about media briefings against her.
Labour MP Toby Perkins, who chairs Ms Kendall's campaign, wrote to the two candidates saying he was "sad" to see negative reports about her based on anonymous briefings.
He said a description of her supporters as "Taliban New Labour" reported in the Daily Telegraph was "inappropriate and offensive", adding: "It was particularly surprising that your campaigns have chosen to do something that, predictably, had the effect of taking precedence over the speeches that both of you were making today."
Mr Burnham and Ms Cooper's teams have distanced themselves from the "Taliban" quote, which was attributed to a source in one of their campaigns.
The contest will involve a series of public and televised hustings over the coming weeks, with the winner announced before the party conference in September.
Mr Corbyn told BBC 2's Daily Politics he "fully acknowledged" that some of his nominations came from colleagues who did not support his candidacy, but who wanted to ensure a full debate.
"I will take part in that debate and hope that at the end of it the Labour Party emerges stronger and more resolute in opposing the principles behind austerity and impoverishment of the poorest in Britain," he said.
The election was sparked by Ed Miliband's resignation in the wake of the party's electoral defeat in May.
Mr Burnham topped the list with 68 nominations, followed by Ms Cooper on 59, Ms Kendall on 41 and Mr Corbyn on 36.
This leaves 28 Labour MPs who did not nominate anybody.
Mr Corbyn's appearance on the ballot paper was criticised by Labour MP John Mann, who tweeted: "So to demonstrate our desire never to win again, Islington's Jeremy Corbyn is now a Labour leadership candidate."
But it was welcomed by other Labour figures who said it would ensure a wider range of candidates.
Former shadow cabinet minister Sadiq Khan - who is running for the London mayor - said he would nominate Mr Corbyn, without voting for him in the ballot, to ensure the "widest possible debate".
Labour MP and another London mayoral hopeful, Diane Abbott, who stood for the leadership in 2010, told the BBC the way the party elected its leaders must change, claiming the 35 MP threshold "artificially narrows the choice".
"And it doesn't just lock out the left, it locks out newer candidates and younger candidates," Ms Abbott said.
BBC assistant political editor Norman Smith said there was some "unease" among MPs - both that the party had rushed into a contest and about the calibre of the line-up.
Former Labour minister Frank Field has written to Labour Party chairman John Cryer to request a rule change to make it easier for the party to get rid of a failing leader.
Mr Field told the Mail on Sunday: "We cannot hide from the fact that we made catastrophic errors in the choice of our two most recent leaders, Gordon Brown and Ed Miliband.
"Having chosen them, it was then impossible to change them when we were hurtling towards election defeat."
He said the Conservative Party "would not have hesitated for a moment" to ditch their leader if they had been in the same "predicament".
Under his proposal, a vote of confidence in the leader would be called if it had the support of 30 anonymous Labour MPs. A vote on his proposed changes is expected next week.
Margaret Hodge, who backs a change in the rules, said that now was the right time to do it, before a new leader was elected.
"Our rules are just too cumbersome, unlike the Conservative Party's," she told BBC2's Daily Politics programme, adding that the aim was to "mimic" the Tories in this area.
Put to her that it suggested a lack of confidence in the current candidates, Ms Hodge disagreed, saying: "This is entirely about putting in place a mechanism that we can use, because of our experience in the past, without actually passing judgement on any individual."
Mr Burnham and Ms Cooper both made speeches on Monday setting our their pitch.
Mr Burnham said he wanted as many people as possible to be involved in the leadership election, saying it had to be "a campaign for Labour to reach out to every corner of the country and win again".
The shadow health secretary also said Labour would need to "look again" at the tuition fees system.
Ms Cooper, shadow home secretary, set out her background as a "comprehensive girl" whose first job was picking fruit on a farm before she went on to secure a place at Oxford University.
She said the UK should invest 3% of GDP on "science, technology and innovation".
The winner will be decided by a vote of Labour Party representatives, members and affiliates, to be conducted on a one-member, one-vote basis.
The result will be announced at a special conference on 12 September.
A vacancy has also arisen for the deputy leadership, after Harriet Harman announced she would be stepping down once the posts were filled.
Nominations for that election close on Wednesday, with seven candidates in the race.
Labour leadership timetable
15 June: Nominations for leader will close at midday
17 June: Nominations for their deputy will close at midday
12 August: Deadline for people to join the Labour Party
14 August: Ballot papers sent out by post
10 September: Polling closes at midday
12 September: Winners announced at special conference
27 September: Labour's party conference begins