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

The raising of the alarm comes 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 previously pledged not to intervene in the leadership contest, now says that he feels he has to speak out about the “car crash and more” Labour might be about to experience.

The ex-No 10 communications chief admits in a blog that Mr Corbyn is a “likeable, sincere and good local MP”, who is “millions of miles away from the detestability of a George Galloway or other lesser known figures on the far Left...”

But Mr Campbell also urges those who have 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 pause for thought and consider Labour’s recent history and the positive changes it has 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,” declares Mr Campbell.

He argues just as Margaret Thatcher loved it when Neil Kinnock was having to spend more energy dealing with the hard Left than he did with her, so the Tories cannot believe their luck at the turn Labour’s election is taking.

“Our job is not to help them do theirs. They have enough advantages already,” Mr Campbell insists.

He argues that “whatever the niceness and the current warm glow”, Mr Corbyn is causing, he will be a leader of the hard Left for the hard Left and will be espousing policies that the public will not accept, particularly in many of those marginal seats Labour will have to win to get back into power.

A Corbyn victory would mean “stand by for chaos, in the PLP, in the party and in the country.”

The former No 10 communications chief notes how it was bad enough trying to sell Ed Miliband on the doorsteps in some marginals, if Mr Corbyn wins, Labour campaigners will not be allowed over the doorstep.

“Some of the positions winning him the loudest applause in his packed meetings are those that will be met with the most deafening silence when campaigners get out on the doorsteps of the undecided come election time. His long career has laid a plentiful minefield for currently quiet Tory researchers and campaigners. The past, he will discover, is not another country,” writes Mr Campbell.

He claims the other three candidates are every bit as important in the leadership debate than Mr Corbyn. “More so in fact, if we are serious about Labour being a party of power rather than just a party of protest that marches, campaigns, backs strikes, calls for ministerial resignations, more money for every cause going, shouts and bawls and fingerjabs but is ultimately powerless in the face of changes the Government is now making, freed from the constraints of coalition, loving the chaos that Labour’s election has unleashed.”

Mr Campbell insists Mr Corbyn cannot win the 2020 General Election and echoes former Home Secretary Alan Johnson’s call for the “madness of flirting with the idea of Corbyn as leader...to stop”.

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

The ex-spin doctor explains that everything he has seen both of leadership and of Labour tells him Mr Corbyn’s ability to lead and hold the party together is likely to be low; his ability to reach those parts of the country Labour has been losing, whether to the Tories, to Ukip or the SNP, will be even lower.

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

While they each could win the next General Election, Mr Corbyn could not.

He adds: “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.”

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."

Labour insists its vetting system is “robust” with extra staff brought in to sift applications. But since the election, it has been estimated that the existing 200,000 party membership has been expanded by around 150,000 by new members and “registered” members, who have paid the £3 to take part in the contest.

On Thursday and Friday, Mr Corbyn 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.