Tony Blair has conceded that Theresa May and the Conservatives are heading for victory at the general election on 8 June. The former Labour prime minister, speaking with just over six weeks to go before the vote, said that his party should urge the electorate to back a strong opposition.

"We know who's going to be prime minister on 9 June, that's not the issue...It will be Theresa May if the polls are right," he told Sky News on Thursday 27 April.

"The real issue is [a] blank cheque: What mandate does [May] claim both on Brexit and the health service, education and all the other things.

"The most powerful argument for Labour in this election, because of the way the polls are and the way the polls are on the leadership issue, is to say that it's important for our democracy that the government is held properly to account and she needs a strong opposition."

The latest intervention from Blair comes after he controversially urged voters to back any candidate who opposed Brexit.

"We risk a Parliament which is lop-sided in its make-up; which has a big Tory majority – in part delivered not because of the intrinsic merits of Brexit or the Tories themselves but because of the state of Labour; where they will claim a mandate to take us wherever they will; when we desperately need representatives who will at least keep an open mind," he told The Guardian.

The former prime minister, who is arguably Labour's most successful leader, has re-entered British public life after unsuccessfully campaigning for a Remain vote at the EU referendum. Blair delivered a speech for the Open Britain group, an offshoot of the Stronger In campaign, after the Brexit vote and established the Tony Blair Institute.

The London-based organisation plans "to make globalisation work for all". But Blair remains a controversial figures inside and outside of the Labour Party, not least over his decision to take the UK to war with the US against Iraqi despot Saddam Hussein in 2003.

Corbyn, who opposed the war, shocked Westminster in 2015 by winning the Labour leadership contest. But the party is now lagging 16 points behind the Conservatives, according to the latest YouGov poll. The survey, of more than 1,500 people between 25 and 26 April, put the Tories on 45% and Labour on 29%.