Britain's opposition Labour Party is expected to crown hard left lawmaker Jeremy Corbyn as its new leader today after a surprise surge in support for his anti-austerity platform, and despite fears that he will make the party unelectable for years.

Corbyn, a 66-year old parliamentary veteran who was relatively unknown a few months ago, has hit upon a strong appetite within his party for a radical alternative to Conservative Prime Minister David Cameron's austerity agenda.

"There has to be public investment in order to bring about a growing economy," he told reporters at a triumphant final campaign rally in London on Thursday. "You can't cut your way to prosperity, you grow your way to prosperity," he added.

The winner of the internal ballot of party members and registered supporters is expected to be announced at around 10.30 am.

The prospect of a return to the party's socialist roots has drawn stark warnings that they will be annihilated in 2020 national elections by a British public that in May re-elected Cameron for a second term on a promise to cut spending.

"The party is walking eyes shut, arms outstretched over the cliff's edge to the jagged rocks below," warned former centrist Labour prime minister Tony Blair in one of three increasingly desperate pleas to voters to stop the Corbyn surge.

But by promising to increase, rather than cut, government investment though money-printing, Corbyn has found favour among disillusioned young voters and socialists who had drifted away from the party after nearly two decades battling for the political centre ground.

That, combined with an opening up of the Labour leadership contest to non-party members, has seen Corbyn pull ahead of his three rival candidates.

He faces two former ministers, Yvette Cooper and Andy Burnham, who both campaigned on moderate fiscal platforms similar to failed centre-left former leader Ed Miliband, and Liz Kendall who sought to emulate Blair's outright centrist pitch.

The likely abandonment of the political centre ground, particularly on the subject of balancing Britain's books, is seen by many as a gift for the Conservative Party that could herald a prolonged spell in power for the centre-right party.

May's election result showed public backing for less state spending and to return the country to surplus by the end of the decade, while Britain's post-war election results show that elections are not won without the support of centre-ground voters.

"Their whole debate seems to ignore what almost every other political party in almost every other country has now grasped," Cameron said on Friday, referring to Corbyn's anti-austerity views.

"It's arguing at the extremes of the debate, simply wedded to more and more spending, more and more borrowing and more and more taxes. And in that regard they pose a clear threat to the financial security of every family in Britain."

With Cameron having already ruled out a third term as prime minister, Corbyn's rise has put the long race to succeed him into focus. Chancellor George Osborne, Cameron's closest political ally, currently leads the pack ahead of colourful London Mayor Boris Johnson and interior minister Theresa May.