Labour leadership challenger Angela Eagle wants supporters of the party to take advantage of a one-off chance to sign up for the party and end Jeremy Corbyn's under-fire leadership.
The leadership race took yet another turn last night when Corbyn was guaranteed a place on the ballot - 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.
Corbyn's fate is therefore set to be decided by the Labour membership, less than a year after they voted him into leadership.
Eagle therefore faces a major battle to overturn Corbyn's support among the grassroots - he secured 59.5% of the vote in the first round after a surge in support from registered supporters last September - although this time round a fee to register to vote will rise from £3 to £25.
Eagle believes that represents a "pretty good investment for the whole future of democracy" and, with a two-day window next week for Labour backers to sign up, has called on them to pay the fee and help "save" the party.
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."
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. If they want to vote they will have to pay the £25.
Labour faces a summer of bruising clashes that threaten to split the party in two after Corbyn was guaranteed a place in the contest.
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 (NEC).
As jubilant supporters cheered 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."
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."
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, Corbyn would almost certainly have been unable to defend his position.
The timetable for the contest will be published on Thursday but reports suggest it will run well into September.
The MP for Pontypridd, Owen Smith, is expected to announce his leadership bid on Wednesday after he tweeted: