The Communication Workers Union has backed left-wing MP Jeremy Corbyn in Labour's leadership contest, saying the "grip of the Blairites" on the party must be "loosened once and for all".

The union's general secretary Dave Ward said Mr Corbyn was the "antidote" to the "virus within the Labour Party".

It made Andy Burnham its second choice.

Another candidate, Liz Kendall, said Mr Ward's reference to a "virus" was offensive and that what was needed was "an antidote to the Tories".

The Transport Salaried Staffs Association also nominated Mr Corbyn, who is already backed by the UK's two largest unions, Unite and Unison.

BBC assistant political editor Norman Smith said the latest union endorsements meant "the momentum is going further and further behind Jeremy Corbyn".

He said Mr Ward's language was perhaps even more significant, adding: "If that is what the future holds for Labour, you sense that it could be heading remorselessly towards another bout of civil war and blood-letting."

The CWU has just under 200,000 members.

Mr Ward said: "We reject the notion that Labour needs to move to the centre ground of British politics.

"The centre ground has moved significantly to the right in recent years."

Image copyright PA Image caption Postal workers are among those represented by the CWU

The union is recommending Angela Eagle for deputy leader, with Caroline Flint as second preference.

Mr Corbyn has gone from being an outsider to a front runner in the contest to replace Ed Miliband, triggering warnings from senior party figures about a shift to the left.

He only made it onto the ballot paper at the last minute with the help of "borrowed" nominations from some MPs who did not plan to vote for him.

One of the favourites for the position of deputy Labour leader, Tom Watson, said Mr Corbyn would struggle to maintain party unity if he becomes leader, given his history of rebelling against the party line in Commons votes.

Mr Watson, a former deputy chairman of Labour, told the Huffington Post Labour members were "aware of this".

"It's their choice," he said.

"Whoever the members select, it's our duty to work with them."

Image caption Liz Kendall said she was "still in this leadership campaign"

Ex-MP George Galloway suggested he would rejoin the Labour Party "pretty damn quick", if Mr Corbyn wins.

Speaking on LBC, the former Respect MP - who was expelled from Labour in 2003 - said: "I think if Jeremy were to win, everyone on the left will rejoin the Labour Party."

But Ms Kendall, seen as the Blairite candidate in the contest, said Labour would be defeated if it went "back to the politics of the 1980s".

Labour leadership contest

Image copyright Getty Images Image caption Liz Kendall has been urged to back Andy Burnham or Yvette Cooper to defeat Jeremy Corbyn

Who are the candidates? Andy Burnham, Yvette Cooper, Jeremy Corbyn, Liz Kendall

Andy Burnham, Yvette Cooper, Jeremy Corbyn, Liz Kendall Dates: Ballot papers will be sent out on 14 August; voting can take place by post or online. They must be returned by 10 September. The result is on 12 September

Ballot papers will be sent out on 14 August; voting can take place by post or online. They must be returned by 10 September. The result is on 12 September Who can vote? All party members, registered supporters and affiliated supporters - including those joining via a union

All party members, registered supporters and affiliated supporters - including those joining via a union What is the voting system? The Alternative Vote system is being used so voters are asked to rank candidates in order of preference

The Alternative Vote system is being used so voters are asked to rank candidates in order of preference How does it work? If no candidate gets 50% of all votes cast, the candidate in fourth place is eliminated. Their second preference votes are then redistributed among the remaining three. If there is still no winner, the third place candidate is eliminated with their second preferences (or third in the case of votes transferred from the fourth place candidates) redistributed. It is then a head-to-head between the last two candidates

At-a-glance profiles of the four contenders

Speaking on the BBC's Newsnight, she said she had more to do to convince people to back her campaign but insisted there was a "long way to go yet".

Mr Corbyn said he was "honoured" to have the two unions' backing.

"Labour is reconnecting with those who believed that the selling off of Royal Mail and of the railways was a bad idea because it based on short-term profits, not long-term public good," he said.

TSSA general secretary Manuel Cortes said: "Jeremy is clearly articulating why we must end the austerity quagmire and that Labour's economic policies must move on from failed neoliberalism."

Image copyright AFP/Getty Image caption Comedian David Walliams is backing Andy Burnham in the Labour leadership contest

Meanwhile, Labour's acting leader Harriet Harman said the party was "very, very vigilant" about preventing non-Labour supporters from registering votes, after reports some were signing up for £3 in order to back Mr Corbyn.

She said: "We have been absolutely determined to ensure that that is not exploited by Tories who have tried to vote, Greens who have tried to vote, various socialist parties who stand against the Labour Party who have tried to vote in the leadership election."

This involved a "very, very detailed verification process of listening in to phone calls", she added.

Mr Burnham has won the backing of comedian David Walliams.

On Twitter, the Britain's Got Talent judge said he had been so impressed when he met the Leigh MP 10 years ago he told him he would one day lead Labour.

Shadow home secretary Yvette Cooper completes the leadership lineup. The winner will be announced at a special conference on 12 September.