Tony Blair today refused to say “hand on heart” that Jeremy Corbyn would be the best person to lead Britain.

The former prime minister Theresa May will be returned to Downing Street after the election, “if the polls are right”, and that the best approach for Labour know was to campaign on the basis of providing a strong opposition.

After provoking some controversy with a newspaper article which appeared to call on voters to back candidates who opposed a hard Brexit, regardless of party loyalty, Blair said he was not telling people not to vote Labour.

“I think the real issue [in this election] is blank cheque. It’s what mandate does she claim, on Brexit and on the health service and all the other things,” he told Sky News.

“If the polls are right, we know who’s going to be prime minister on June 9. That’s not the issue.”

“I think the most powerful argument for Labour in this election – because of the way the polls are, and the way the opinion polls are and the leadership issue – the most powerful argument for Labour is to say it’s important for our democracy that the government is held to account and needs a strong opposition,” he added.

Blair also warned that leaving the single market, as the Tories are leading Britain towards in their Brexit strategy, was like falling out of the elite champion’s league into league one.

“I think the big missing question that has got to be there is a question that might sound extraordinarily technical but is absolutely fundamental to the future prosperity of this country, which is in the Brexit negotiation, are we taking membership of the single market and the customs union off the table, which I think we probably are.

“If we are and we’re going for a free trade agreement, rather than membership of the single market, that’s a massive for the prosperity of the British economy. That’s why our currency is down 15 per cent.”

“What the Tories have done very cleverly is give people the national interest reason for voting Tory.”

“What they’ve said is, the patriotic thing irrespective of your politics is to vote Conservative because she needs a strong hand in the negotiation, which is a very persuasive argument, until you realise that she’s tied her hands in the negotiation, the prime minister, by saying, ‘I’m not going to put the single market on the table’.”

“People have really got to understand the difference between the single market and a free trade agreement. One’s the champion’s league, the other’s league one.”