Brexit spokesman Sir Keir Starmer is the bookies' favourite to succeed Corbyn

Harriet Harman has backed Labour’s Brexit spokesman Sir Keir Starmer as the successor to beleaguered party leader Jeremy Corbyn, a leaked tape recording has revealed.

At a private meeting of activists, former deputy Labour leader Ms Harman presented Sir Keir, a member of the Shadow Cabinet, as the party’s saviour.

She was cheered as she praised his ‘wisdom, expertise and ability to solve insoluble problems’. She added: ‘It is a miserable time for Labour, but when we hear Keir speak, we feel optimism in difficult times.’

Brexit threatened to ‘divide Britain and destabilise Labour’, said Ms Harman, ‘so it’s just as well we’ve got Keir’.

Sir Keir, the bookies’ favourite to take over from Mr Corbyn, made a passionate cry at the meeting for unity. He said Labour faced a massive fight to survive and he was ready to play his part.

Labour had lost two Elections and ‘must not lose’ the next one, due in 2020. ‘We’d be letting too many people down,’ he said. Being in Opposition was ‘pants’ said Sir Keir. Labour ‘looked divided’ and could achieve nothing from its current position.

In a clear jibe at Mr Corbyn’s Left-wing stance, Sir Keir said Labour could throw Theresa May out of No 10 at the next General Election, but only if ‘we speak again with the right voice for the whole country’.

Harman admitted it was a 'miserable time' for the Labour party but said that Sir Keir brought her 'optimism in difficult times'

Sir Keir wanted Labour back in power soon, but it had to show unity, he said to cheers at the meeting in London six weeks ago.

The leak of the ‘Keir leadership’ recording follows reports that Mr Corbyn may quit as leader over Labour’s slump in the opinion polls, which has been fuelled by its damaging split over Brexit. Moderate Sir Keir has emerged as the front-runner to become the next Labour leader, narrowly ahead of Left-wing MP Clive Lewis.

Sir Keir, former head of the Crown Prosecution Service, has won plaudits from senior party figures such as ex-Lord Chancellor Lord Falconer, a close friend of Tony Blair.

Allies claim his ‘deft’ handling of Brexit, which has seen him maintain his pro-EU stance while pledging to uphold the referendum result and continue his tough questioning of Mrs May’s approach, has shown leadership qualities lacking in Mr Corbyn.

Rumours have been circulating that Corbyn might stand down as party leader over Labour's slump in opinion polls and its damaging Brexit split

But critics say his pro-EU views mean he is out of step with working- class Labour voters who backed Brexit in large numbers. Others say he lacks charisma.

One former Labour Minister said: ‘Keir has all the right ideas, but he lacks a touch of stardust.’ Some Labour MPs believe Mr Corbyn may be forced to quit if the party loses upcoming by-elections in Copeland, Cumbria and Stoke- on-Trent Central on February 23.

Ms Harman denied she was endorsing Mr Starmer as Mr Corbyn’s successor. Mr Starmer was unavailable for comment.