Labour leadership contest: David Miliband backs Kendall

Liz Kendall is one of four contenders for the Labour leadership

Former Foreign Secretary David Miliband has said he is backing Liz Kendall in the Labour leadership race.

Mr Miliband is also the latest party figure to warn against electing left-wing frontrunner Jeremy Corbyn.

Former party leader Neil Kinnock said Labour supporters must choose between "seriously" contesting elections and being in "perpetual demonstration".

Leadership candidate Andy Burnham said he would offer Mr Corbyn a role in the party if he won the contest.

But Yvette Cooper said Mr Burnham should withdraw from the race if he was not prepared to oppose the left-winger.

• Everything you need to know about the four candidates hoping to become the next Labour leader

Writing in the Guardian, Mr Miliband - brother of former party leader Ed - said Mr Corbyn would take the party "backwards" and warned that the "angry defiance" of his campaign would lead only to electoral defeat.

He said he was backing Ms Kendall - who is on the right of the party - as leader, but she herself has acknowledged that she is trailing in the polls.

Lord Kinnock told BBC Newsnight that being a party of demonstration was "fulfilling and noble, but rarely effective".

David Miliband wrote about Ms Kendall's "plain speaking, fresh thinking and political courage"

"I can see why people are angry and want to protest," he said.

"But then they've got to make a decision on whether they want to be part of a labour movement that produced a political party to seriously contest for democratic power - or they want to be in perpetual demonstration."

Meanwhile, Mr Burnham said only he could beat Mr Corbyn and unite the party - but he praised his opponent and said he shared many of his views.

Ms Cooper and Ms Kendall have both said they would not serve in a Jeremy Corbyn shadow cabinet, arguing that policies such as renationalising utilities and scrapping Trident were not credible.

But Mr Burnham has tried to strike a more consensual tone, amid speculation he is going after the veteran Mr Corbyn's second preference votes.

Mr Corbyn said he welcomed Mr Burnham's "inclusive tone".

'Fighting each other'

In a speech in Manchester, Mr Burnham said he shared a "good deal of common ground" with Mr Corbyn on issues such as transport and education, and would offer him a role in rebuilding the party.

But he criticised the Islington North MP's sceptical stance on the European Union and attacked his economic policies, which he said would "leave us open to losing the argument on the economy on day one".

He added: "I won't let our party repeat the history of the early 1980s when we were more interested in fighting each other and we left the pitch clear for Margaret Thatcher to bulldoze her way through Labour communities up and down the country."

He also told supporters he would oppose the government's controversial Welfare Bill if he wins the contest, saying he only abstained in a Commons vote on the issue - a move which attracted criticism from Mr Corbyn's supporters - to preserve party unity.

Mr Corbyn said he welcomed Mr Burnham's olive branch, saying: "The view is mutual - if we win we would involve Andy in our team if he was willing. From day one, whoever wins must pull the party together."

Labour leadership contest

Leadership contenders: Liz Kendall, Andy Burnham, Yvette Cooper and Jeremy Corbyn

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

• Dates: Ballot papers were sent out on 14 August; voting can take place by post or online. They must be returned by 10 September. The result is announced on 12 September

• Who can vote? All party members, registered supporters and affiliated supporters - including those joining via a union. More than 160,000 people signed up to vote as supporters, full members or union affiliates in the final days before the registration deadline, bringing the total size of the electorate to 610,000

• What is the voting system? The Alternative Vote system is being used, with voters asked to rank candidates in order of preference

• How does it work? If no candidate wins outright with more than 50% of first preferences, whoever is in fourth place drops out and the second preferences of their backers are reallocated to the other candidates. If there is still no winner the third placed candidate is then eliminated with their second preferences similarly reallocated. The candidate who has accumulated the most votes through the different rounds then wins.

A spokesman for Ms Cooper's campaign said Mr Burnham was sending a confused message.

"Andy needs to show some leadership and be clear whether he opposes Jeremy or not," he said.

"If he isn't prepared to offer an alternative to Jeremy, he needs to step back and leave it to Yvette."

Ms Cooper and Ms Kendall have rejected claims they considered quitting the contest and throwing their weight behind Mr Burnham in an effort to stop Mr Corbyn winning.