General Election: Theresa May's campaign targets majority in Wales

"Only a vote for the strong and stable leadership of Theresa May on June 8 will deliver a Brexit deal in Britain's national interest", he said.

Speaking to Sky News, he said: 'If the polls are right we know who is going to be Prime Minister on June 9, that's not the issue.

As the party prepared to unveil its Brexit policy on Tuesday, its Brexit spokesman Keir Starmer said a Labour government would discard Prime Minister Theresa May's "reckless" approach to leaving the EU.

He has partly blamed David Cameron's decision to take part in debates ahead of the 2010 general election for enabling the Liberal Democrats to surge and costing the Tories a majority.

Green Party co-leader Caroline Lucas said Labour's "hollow words" on retaining access to Europe's single market after Brexit "simply doesn't stack up".

"We are confident that we can win this election once Labour's message is clearly heard".

Labour's opponents urged them not to be "afraid" of debating them.

Asked whether Mr Corbyn would take part in a broadcast which did not involve Mrs May, he said: "Obviously, if you are talking about a debate about the possible outcomes of the election, you are talking about a debate between the Conservative Party and the Labour Party first and foremost".

While Mr Corbyn's supporters hope his populist appeal as an outsider could lead to an unexpected shock win, similar to the Brexit vote and the US Presidential victory, he now lies very far behind Theresa May in opinion polls.

"So on day one of a Labour government, we will immediately guarantee that all European Union nationals now living in the United Kingdom will see no change in their legal status as a result of Brexit, and we will seek reciprocal rights for United Kingdom citizens in the European Union", he added.

Blair, who was elected PM three times as Labour leader, has positioned himself to join forces with Lib Dems against May, indicating a comeback to politics.

In a speech in London, Mr Starmer said a Labour government would have a "presumption that any new powers that are transferred back from Brussels should go straight to the relevant devolved body".

"Every vote for me is a vote for a strong economy with the benefits felt by everyone across the country", she said. She answered that she will instead be traveling around the country and that her weekly exchanges with Corbyn in the House of Commons had provided ample opportunity to see the two in action.