Tony Blair has predicted Theresa May will win a “large majority” in the general election but called for a “movement” of pro-EU voters to prevent her getting a mandate for a “Brexit at any cost”.

The former prime minister said the opinion polls, which put Labour as many as 24 points behind the Tories, “appear to be definitive” in pointing towards a big Tory victory on June 8.

But he called for a “movement of informed voters” to demand candidates make clear whether they would support the UK leaving the European Union with no deal, or one that does not give good access to the single market.

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A failure to do so would give Mrs May, who has said “no deal is better than a bad deal”, a “free hand” to pursue a “Brexit at any cost”, with no option for the UK to change its mind about leaving the EU if the exit terms are unpalatable.

Writing in the Evening Standard, he said: “Whatever may be desirable, the polls appear to be definitive on the election and the respective polling positions of the leaders of the Government and Opposition.

“There are many great Labour candidates and MPs and I will be fully supportive of them. But the fact is that if the polls are right, Theresa May will be PM on June 9 with a large majority.

Tony BlairTony Blair has called for a “movement of informed voters” to demand candidates make clear whether they would support a hard Brexit ( Victoria Jones/PA)

“But with what mandate? The British people have to navigate a way through not just choosing a government but working out what they want the governing mandate to be.

“Unless there is some clarity around that mandate, a re-elected Tory Government could do what it wants. But is that what we want? The PM says she wants her hand strengthened in the Brexit negotiations. This is plausible as a headline but requires greater interrogation. What she really wants is a free hand – but to do what?

“Some say it is to defeat the Tory Right so that she can go for a ‘softer Brexit’. This is naive. The opposite is true. At present, if she wanted to face down the Tory Right she has a Parliament with a majority to do so. What she doesn’t have is a Parliament that would vote for Brexit at any cost.”

A broken Union flag cake on the backdrop of the EU flag(Yui Mok/PA)

Mr Blair said the Daily Mail’s “Crush the Saboteurs” headline the day after Mrs May called the election revealed her true intentions.

Calling for a mobilisation of voters to demand clear statements of candidates’ positions on Brexit, and particularly whether they would refuse to support an exit that “that substantially diminishes our access to the single market” or a no deal outcome, he added: “We don’t know what size majority Mrs May will get. But we can determine what mandate she can claim.”