Britain’s Opposition Labour Party yesterday began voting for a new leader in a contest that polls indicate will be won by Jeremy Corbyn, a veteran fan of Karl Marx who has upstaged rivals by promising a radical shift back to the party’s socialist roots.

The victor will have to marshal supporters in a referendum on EU membership and is likely to be the main challenger to Prime Minister David Cameron’s Conservatives in the 2020 election.

After Labour’s worst election defeat since 1987 in May, four contenders are battling in a selection process characterised by Labour’s most successful leader, Tony Blair, as a tug of war between the party’s head and its heart.

On one side are those who say that a left-wing outcry over the Conservative government’s spending cuts shows Labour should move closer to its founding principles, rooted in the socialist and trade union movements of the early 20th century. On the other side are those who insist the party can only hope to win in 2020 if it reclaims the centre ground from which it enjoyed its most prolific election success.

Tony Blair says Labour faces annihilation if it picks Corbyn

Blair, who won the 1997, 2001 and 2005 elections, has said Labour faces annihilation if it picks Corbyn.

The most “Blairite” candidate, Member of Parliament Liz Kendall, has also warned of electoral oblivion if Corbyn wins. But supporters look set to reject a return to Blair’s ‘New Labour’ centrist stance which embraced closer ties with business while increasing spending on health, education and welfare.

Kendall lags in fourth place according to polls after struggling to sell her vision of a Labour that espouses fiscal discipline. The remaining two candidates, former ministers Andy Burnham and Yvette Cooper, are campaigning from a more orthodox centre-left position, seeking to broaden the party’s appeal.

Labour Party leadership candidate Liz Kendall joins children on a bouncy castle during her visit to a Kidszone project in Manchester, Britain, yesterday. Kendall has warned of electoral oblivion for UK Labour if Jeremy Corbyn wins the leadership race.

Both campaigns are based on a promise to win back voters who ditched them in May for either the perceived economic competence of Cameron’s centre-right Conservatives, the surge in nationalist sentiment in Scotland or the anti-European Union appeal of the UK Independence Party (Ukip).

The winner will be announced on September 12 after a ballot of more than 600,000 voters, made up of party members, affiliated trade union members and other registered supporters.

The Guardian, the main left-leaning broadsheet newspaper, endorsed Cooper, while the Daily Mirror, a Labour-supporting tabloid, came out in support of Burnham.

But Corbyn’s calls for renationalisation and more government spending have thrilled party activists and Labour’s trade union backers, fuelling the rise of the 66-year-old Member of Parliament from rank outsider to bookmakers’ favourite.

Usually seen in a rumpled jacket and open-necked beige shirt – complete with white vest showing underneath – Corbyn has sought to energise disillusioned voters with a promise of a radical alternative to centrist politics. Polls show that as ballot papers begin arriving in e-mail inboxes and dropping onto doormats, around half will be returned with backing for anti-war protester Corbyn, and his policies of higher taxes for businesses and the rich.

The prospect of a Corbyn victory has prompted warnings the party is repeating mistakes of the 1980s when a left-wing insurgency split Labour in two and set up a humiliating 1983 election defeat.

Fears that the current leadership contest has been hijacked by hard-left sympathisers and rival political parties have led some Labour lawmakers to call for the vote to be suspended.