London - Britain's Labour opposition leader Jeremy Corbyn has been propelled into an unprecedented position of power over Brexit but needs to walk a tightrope in dealing with a government his party despises, analysts said.

Unable to get the EU divorce deal through parliament with the votes of her own Conservatives and their right-wing allies, this week Prime Minister Theresa May dramatically changed course and turned to veteran socialist Corbyn.

But by jumping into bed with May, Corbyn risks being seen as the enabler of a Conservative Brexit -- anathema to many of his supporters.

Put simply, the bind for Labour is that it has many Remain-backing MPs representing Leave-supporting provincial seats and is led by a lifelong eurosceptic elected by pro-EU metropolitan types.

Labour's Brexit policy thus far has been one of constructive ambiguity, trying to retain its Leave-voting heartlands while not alienating their Corbynist, Remain-voting new party members.

By accepting the invitation to seek common ground with political enemy May, it could be decision time for Corbyn -- something which risks alienating supporters either way.

- Splitting the Conservatives -

"Participation and then walking away is his best option," said Steven Fielding, a professor of political history at the University of Nottingham.

"If they come to an agreement, that's quite dangerous for him. He cannot have his cake and eat it," he told AFP.

Corbyn is no fan of the European Union, seeing its free-market fundamentals as a roadblock to a socialist society.

Theresa May's premiership and Brexit, since 2016

The 69-year-old swept to the Labour leadership in 2015 on the back of thousands of new, young party members enthused by his undiluted leftist outlook.

The challenge for him is to retain that support base whilst upholding Labour's pledge to respect the verdict of the 2016 referendum vote to leave the EU.

He and May had preliminary talks on Wednesday and their negotiating teams were back in a third day of urgent discussions on Friday. Labour thus far wants a much softer form of Brexit than May.

- Premiership prize -

Britain is also now seeking an extension to its EU membership from April 12 to June 30.

EU officials are saying the deadline may have to be much longer -- up to a year -- but could be shortened.

Unlike Corbyn, May has nothing to lose: she will step down after seeing Brexit through.

"Corbyn will try to get as much kudos from the negotiations... paint himself as a national figure and potential prime minister," Fielding said.

"His participation has already caused May all kinds of grief," he said, referring to the outrage the talks have triggered among right-wing Conservative Brexiteers opposed to the prime minister's divorce deal they see as too soft already.

"He might have already further engineered Conservative divisions, which are helpful. By participating in these talks, he might hope to have gained something -- and then walk away."

The vast majority of Remain-backing Labour MPs want a second Brexit referendum, seeing this as a legitimate route towards keeping Britain in the EU.

However, Corbyn is not warm to the idea and parliament's lower House of Commons, as a whole, is against it.

"It's been very difficult for Jeremy Corbyn but now he is really in a position where he will have to make some big decisions very quickly," said Paul Breen, a senior lecturer at the University of Westminster.

"Corbyn is trying to argue for some kind of sensible compromise."

Anand Menon, a European politics professor at King's College London university, said May and Corbyn did have some common interests.

"Neither of them wants a referendum. Neither of them wants a long delay, and neither of them are particularly keen on holding European Parliament elections" -- something that must happen if Britain stays in the EU beyond May 22.

How does a country leave the European Union?

Meanwhile Alastair Campbell, who was centre-left Labour prime minister Tony Blair's communications chief and is now a key figure in the campaign for another referendum, said May had made Corbyn the most powerful opposition leader of modern times but urged him not to "play her game and help her carry a disastrous Brexit over the line."

Pressing Corbyn to demand a second vote, he wrote in The Daily Telegraph newspaper: "The Labour leader should make the most of it. It might just make him prime minister."

By Robin Millard