Jeremy Corbyn and Owen Smith will face each other in the final Labour leadership hustings live on Sky News tonight.
It has been a bitter campaign marred by internal strife, court cases and division in the party - and the debate could prove decisive, with hundreds of thousands of votes still to be cast in the contest.
The 200-strong audience in London will be balanced equally between Corbyn supporters, Smith supporters and undecided Labour supporters, with just one week to go until the ballot closes.
It was a contest triggered by a revolt of Labour MPs against the leadership of Jeremy Corbyn in the aftermath of the vote to leave the European Union.
Although Mr Corbyn won by a landslide in last year's leadership election, he has struggled to win over elected Labour MPs.
His campaign has sought to portray Mr Smith, who served until June as the shadow work and pensions secretary, as disloyal to the elected leadership.
Meanwhile, Mr Smith has focused on his rival's performance during the EU referendum campaign and has promised voters a chance to revisit the decision to leave the trading bloc.
The incumbent leader is clear favourite to be re-elected, but only one public poll has been published of Labour members.
Both candidates face a nervous wait before the result is announced ahead of the Labour Party conference in Liverpool on Saturday 24 September.
:: A special one-hour documentary on the contest will air on Sky News at 8pm tonight, immediately before the debate begins at 9pm.