Leadership challenger Angela Eagle has pleaded with Labour backers to "save" the party and "heal" the country by signing up for a vote to kick out Jeremy Corbyn.
Labour faces a summer of bruising clashes that threaten to split the party in two after the leader was guaranteed a place in the contest.
But Ms Eagle urged his opponents to take advantage of rules that mean they have two days next week to become registered supporters, a one-off status that gives them a vote, if they pay £25.
She told BBC Two's Newsnight: "I would say to the nine million Labour voters out there, there's two days next week when you can actually pay £25, help save the Labour Party, make our democracy work and help me heal our country. Join as a registered supporter.
"Join us in this battle, let us win the Labour Party back."
Ms Eagle said the fee was a "pretty good investment for the whole future of democracy".
Mr Corbyn said MPs should come together after it was agreed to automatically include him on the ballot following six hours of fraught discussions by the ruling National Executive Committee.
The decision is a major blow to the majority of Labour's MPs who are desperate to overthrow the leader they say does not have what it takes to lead them to power.
As jubilant supporters cheered Mr Corbyn, he told reporters: "I'm sure Labour MPs will understand that the party has to come together in order to present to the British people the options of a different and better way of doing things."
Pledging to campaign "on all the things that matter", the leader said anyone who had any disagreements should "come and talk about it".
He added: "I have been elected, last year, 10 months ago today, with a very large mandate. I respect that mandate. It's a responsibility I'm carrying out.
"I would hope there isn't going to be a legal challenge. There's been a very long legal discussion this afternoon. There were very well-qualified lawyers on hand to advise, so I think we are fine."
NEC members wrestled with legal advice over whether the leader would need to secure 51 nominations to make it on to the ballot after both sides insisted Labour's rulebook backed their case. The committee favoured his position by 18-14.
Without automatic inclusion in the race Mr Corbyn would almost certainly have been unable to defend his position.
Ms Eagle now faces a major battle to overturn his support among the grassroots. When Mr Corbyn was elected leader, he secured 59.5% of the vote in the first round after a surge in support from registered supporters.
Labour Party members need to have signed up on or before January 12 to be eligible to vote in the leadership contest. It means more than 100,000 people who have signed up since then will not be sent ballot papers.
Registered supporter status, which gives people a one-off vote if they pay a fee, will cost £25 this time round instead of the £3 payment that saw thousands of Mr Corbyn's supporters sign up in the run-up to last September's vote.
The timetable for the contest will be published on Thursday but reports suggest it will run well into September.
A Labour spokesman said: "The timetable for the contest for the leader of the Labour Party has been agreed by the NEC and will commence on Thursday. The timetable will be published then."