Jeremy Corbyn says he cannot give an ironclad assurance Brexit would proceed if he were unhappy with the EU agreement.
But Labour remains very much the underdog in next month's election.
He may be the firm underdog in the lead-up to Britain's June 8th poll, but, at Labour's campaign launch in Manchester, Jeremy Corbyn was lapping up the adulation.
Acclaimed British actress Julie Hesmondhalgh introduced him to the stage.
"And now, everyone, please give a warm and riotous Manchester welcome to a man who has dedicated his life to giving a toss* about other people, Jeremy Corbyn!"
The Labour leader has told the party faithful the Westminster system of democracy is broken and the economy is rigged.
"Both are run in the interests of the few. Labour is under attack because we are standing up to the elites who are determined to hijack Brexit and pay even less tax and take even more of the wealth that we all create."
And while he declared the Brexit issued settled, he appeared to indicate otherwise in a later interview with the BBC.
(Corbyn:) "Look, it was a clear vote in the referendum a year ago, but there is now the negotiations, which have already begun. I sent a letter to President-Elect Macron last night congratulating him on his election and also setting out in broad terms what our aims are in these negotiations. To have good relations with Europe, of course. Secondly, to make sure there is a trade access, a tariff-free trade access, to European markets."
(Interviewer:) "But on that specific point, whatever happens in the negotiations, however well or badly they go, we would be leaving if you were prime minister."
(Corbyn:) "Look, we will go into the negotiations with the determination to achieve what I've just outlined."
Meanwhile, Prime Minister Theresa May has been campaigning in the city of York.
"Thank you very much, everybody, and it's great to be with you here today in York."
While her Conservatives continue to hold a commanding lead in polls, Ms May was keen to push the idea of Labour winning with help from Liberal Democrats and Scottish nationalists.
"And a Jeremy Corbyn-led coalition of chaos is a real possibility. We must not let that happen. Now just picture him sitting at the negotiating table with the combined might of the European Commission and 27 other European countries ranged against him ... And we must take absolutely nothing for granted. "
She has seized on Emmanuel Macron's decisive victory in France as evidence that Britons must vote in force for the return of her Government.
"The French president, the new French president, was elected with a strong mandate that will put him in a strong position in the negotiations. So we need a strong mandate to put us in a strong position in those negotiations."
Polls are showing she will get that strong mandate, although the latest survey conducted for The Independent newspaper has the Conservative lead down by eight points.
Labour support is up four points to 28 per cent, still well behind the Conservatives on 44.
