LABOUR could be “finished” if Jeremy Corbyn wins the Labour leadership election, Alastair Campbell, Tony Blair’s former spin doctor, has warned.

The warning came as senior Labour backbencher Barry Sheerman called for the contest to be “paused” over fears it has been infiltrated by people from other parties.

Mr Campbell, who had previously pledged not to intervene in the leadership contest, said he felt compelled to speak out about the “car crash and more” that Labour was facing.

The ex-No 10 communications chief urged those who had joined the party to make the Islington North MP leader, believing him to be “some kind of cross between Russell Brand, Nicola Sturgeon and their favourite uncle”, to consider Labour’s recent history and the positive changes it had made to the NHS and the welfare system as well as creating the Scottish Parliament and introducing the minimum wage.

“The Labour Party, if it elects Jeremy Corbyn as leader, is selecting someone that every piece of political intelligence, experience and analysis tells you will never be elected Prime Minister,” declared Mr Campbell.

He argued if Mr Corbyn won on September 12, he would be a “leader of the hard Left for the hard Left” and would be espousing policies the public would not accept, particularly those in many marginal seats Labour needed to win to get back in power.

The former Downing Street media chief claimed Mr Corbyn simply could not win the 2020 General Election and echoed former Home Secretary Alan Johnson’s call for the “madness of flirting with the idea of Corbyn as leader...to stop”.

He stressed: “That means no first preferences, no second preferences, no any preferences. It frankly means ABC; Anyone But Corbyn.”

Mr Campbell insisted the other three candidates - Yvette Cooper, Andy Burnham and Liz Kendall - needed to show they were not just in a fight for the leadership but “to save the party”.

He added: “With Corbyn, I’m afraid I can see only the route to defeat, and much, much worse. I wish it wasn’t so. But it is. And it is horrible to watch unless you’re David Cameron or George Osborne; as things stand his likeliest successor in No 10.”

Liz Kendall, regarded as the Blairite candidate in the contest, echoed Mr Campbell’s concerns, saying: "I don't think Jeremy's policies are right for 2015, let alone 2020 or 2025."

Meantime, Mr Sheerman said those registering to take part in the leadership contest included members of the Socialist Workers Party, the Green Party, the Conservatives and Ukip; insisting it should be “paused”.

But Diane Abbott, one of Mr Corbyn’s supporters, said her colleague’s call for a halt was “ridiculous”.

She said such a suggestion was coming from people, "who think their side will lose", adding: "This election is being fought under rules that were agreed by the whole party last year."

It is thought that around 190,000 of the 390,000 people who are eligible to vote in the leadership election have signed up since the party's General Election defeat in May. So-called “registered supporters” have to pay just a £3 fee to take part in the contest. Labour insists its vetting system is “robust” with extra staff brought in to sift applications

Elsewhere, Mr Burnham warned the Prime Minister not to try to "bounce" Labour into supporting military action in Syria.

Ministers have raised the prospect of air strikes on so-called Islamic State extremists in Syria but have made clear this would only happen with the Commons' approval.

MPs rejected air strikes in Syria in 2013 but have since backed action in Iraq.

Mr Burnham said he "wouldn't rule it out" but was "struggling" with whether or not action in Syria would be legal.

On Thursday and Friday, Mr Corbyn, who last night launched his youth policy in London, will be in Scotland with rallies planned for Aberdeen and Dundee on Thursday and Edinburgh and Glasgow on Friday. The Glasgow event has already sold out.