Labour Party leadership rivals Jeremy Corbyn, left, and Owen Smith © EPA

Jeremy Corbyn’s opponents have claimed he is on course to lose the Labour leadership election, arguing he has been deserted by tens of thousands of unionists and registered supporters over the past year.

Defeat for Mr Corbyn would upend conventional political wisdom, with bookmakers ranking him as 1-8 odds-on favourite. No polling of eligible voters has been published since Owen Smith emerged as the sole challenger a month ago.

Saving Labour, an anti-Corbyn group, said Mr Smith would win just over 50 per cent of the vote, which started this week. It said it had signed up 70,000 registered supporters, who can vote by paying a one-off £25 fee; that number would account for more than one-tenth of eligible voters. However, its overall forecast is based largely on assumptions and anecdotes.

“There’s a lot of people who now see the leadership in a completely different way to how they did last year,” said Reg Race, a spokesman for Saving Labour. “Nobody last year challenged the Momentum/Jeremy approach to recruiting people. We have.”

The figures are an attempt to counter the aura of inevitability around Mr Corbyn, who has addressed rallies of thousands of supporters in cities including Sheffield, Liverpool and London.

The only published poll, carried out by YouGov in July, showed Mr Corbyn with a sizeable lead over Mr Smith, who at the time was still one of two challengers. Joe Twyman, YouGov’s head of political and social research, said that based on news coverage since then, his sense was that Mr Corbyn remained “in pole position. But there’s no guarantee”.

Mr Corbyn’s campaign did not respond to requests for comment. He has also received the nomination of Britain’s two largest trade unions, Unite and Unison, and the majority of local Labour parties that have so far declared. Momentum, the group formed following his leadership campaign, has remained active online and with grass-roots meetings.

On Wednesday the Labour leader laid out his plans to end private sector outsourcing and financing in the NHS, while Mr Smith pledged he would fight for a second referendum or a general election on the terms of Brexit.

140,000

Recent members excluded from the vote

Ballots are being issued this week to 647,000 Labour members, registered supporters and affiliated supporters. Last year, when the eligible electorate was 554,000, Mr Corbyn won 59.5 per cent of the votes. Some critics fear he could achieve a higher proportion of the ballots this time, giving him the political capital to reshape the party’s executive and parliamentary party.

However, Saving Labour argues that the electorate has changed notably in the past 12 months, thanks to an anti-Corbyn sign-up drive and concern around the Brexit vote.

In particular, Mr Corbyn’s 2015 victory was helped by winning 84 per cent of the registered supporters, who paid a one-off £3 fee to be able to vote in the contest. In the current contest, previously registered supporters are not eligible unless they pay the separate £25 fee.

Although Mr Corbyn is bookmakers’ heavy favourite, his odds have lengthened slightly after a court ruled the Labour party acted legally in excluding around 130,000 new members from voting. Those members were thought to be largely favourable to Mr Corbyn, based partly on the reasons that some of them gave for joining.

The result of the election will be announced on September 24 at Labour’s autumn conference.

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