The Labour party will this week fire the starting gun on the race to replace Angela Rayner as its deputy leader, in a contest that is set to amplify the leftwing's dissatisfaction with Prime Minister Sir Keir Starmer.
The party's national executive committee is expected to meet on Monday to agree the timetable for the race, which was triggered by Rayner's resignation over the underpayment of stamp duty on Friday.
Her departure leaves an empty podium that several left-leaning MPs are keen to occupy. But the process to select a deputy for the Labour leader will expose splits inside the ruling party on issues from the war in Gaza to wealth taxes as its left and right battle for the support of hundreds of thousands of members.
That may prove an unwelcome distraction for Starmer -- whose poll ratings have plummeted -- as he tackles irregular migration and prepares for a tax-raising Budget and a state visit by US President Donald Trump.
"The deputy leadership contest will become a referendum on Starmer's government -- and inevitably will elect the most critical candidate, unless [the leadership] can arrange a coronation," said one MP. "If the members could get shot of Keir right now, they would be over the moon."
The race has the potential to replicate the hard-fought deputy leadership contest in 1981 between the centrist incumbent Denis Healey and his leftwing challenger Tony Benn, which was dubbed a "battle for the soul of Labour". Healey won that bitter contest with only a whisker over 50 per cent of the votes.
Paul Richards, a veteran Labour adviser, said: "The danger is that Labour uses the deputy leadership contest, not as a vote of confidence in the leadership, but as an emotional spasm and moment of virtue signalling. The last thing we need is a rerun of Benn/Healey, ripping the party apart."
Nearly half a century later, many MPs believe a Starmer loyalist would struggle to win the crown.
Some party members who voted for Starmer as leader five years ago have been dismayed by his subsequent shift to the right, aimed in part at stemming the surge in public support for Nigel Farage's Reform UK.
Many on the Labour left want chancellor Rachel Reeves to introduce a wealth tax or relax her borrowing rules, and strongly oppose spending cuts.
Whoever wins the title will lack Rayner's status, given that Starmer has already given her job as deputy prime minister to David Lammy. However, the role provides a public platform and a seat on the party's ruling national executive committee.
This means the winner could become a lightning rod for dissent within the Parliamentary Labour party. Starmer's team is already nursing its wounds after a surprise rebellion by more than 100 MPs earlier this year forced him to drop plans to cut £5bn in welfare spending.
Although no MPs have announced their candidacy, potential challengers could include Dame Emily Thornberry, chair of the House of Commons foreign affairs select committee, former transport secretary Louise Haigh, high-profile Walthamstow MP Stella Creasy or doctor Rosena Allin-Khan, who came second behind Rayner in 2020.
The NEC meeting on Monday is expected to agree the logistics of the impending race. Under party rules, candidates must obtain nominations from 20 per cent of MPs -- about 80 -- to stand.
They must also win the backing of either 5 per cent of local Labour parties or three affiliates, two of which must be trade unions. Those who get through this stage are then put before a vote of the party membership.
Despite speculation by MPs that Starmer would try to delay the contest for as long as possible, allies of the prime minister insisted that he wanted it wrapped up before Reeves delivers the Budget on November 26.
Some leftwingers are nervous of a stitch-up designed to ease a loyalist into the seat.
John McDonnell, former shadow chancellor under Labour's previous hard left leader Jeremy Corbyn, said: "It's crucial the election is used to unite the party and so it has to be an election where candidates from the broad church of the party are enabled to stand."