Alan Johnson will hit the campaign trail with Yvette Cooper next week in a last-ditch bid to stop the surge in support for Jeremy Corbyn.
The former home secretary is one of the highest-profile Labour MPs and his much-anticipated endorsement of Cooper as party leader is a huge boost to her campaign.
Johnson will host Cooper at a major rally in his constituency of Hull at the end of next week, BuzzFeed News understands. The event, which will also be attended by Labour frontbencher and local MP Diana Johnson, is aimed at ramping up support for Cooper before ballot papers are sent out on Friday 14 August.
It is expected that most Labour members and supporters will vote within days of receiving their ballot paper, so the next 10 days of campaigning are crucial. The result will be announced on 12 September.
Cooper has struggled to keep up the momentum in the four-month leadership contest against rivals Andy Burnham and veteran left-winger Corbyn, who is enjoying a flood of support from disillusioned voters.
As of Tuesday afternoon, bookmakers William Hill had Cooper at 11/4 to win, with Corbyn at 6/4 and Andy Burnham the favourite at 11/8. Liz Kendall had fallen back to 50/1.
A source close to Cooper said she was "very pleased with the support from one of the Labour party's most senior and insightful politicians".
Johnson is a former postal union leader who served in a string of cabinet posts under Tony Blair and Gordon Brown. He is widely respected across the party, with many having privately called for him to step up to the plate himself.
He said Cooper was the "most qualified candidate to lead our party back to government" and also "happens to be a woman", after a over a century of male leaders. "Let's end the madness and elect her," he wrote in The Guardian.
Johnson said he had "never had the ambition or the appetite that this job requires. Neither has Jeremy Corbyn – as he's honest enough to admit."
He said Corbyn had been "cheerfully disloyal to every Labour leader he's ever served under. That's fine so long as members understand that it's the loyalty and discipline of the rest of us that created the NHS, the Open University and all the other achievements I've mentioned and the many that I haven't."