Theresa May is on course to win a majority of more than 100 in a June snap election, with Labour’s pro-Brexit voters deserting Jeremy Corbyn, according to polling data for The Times.

The prime minister ripped up her promise not to hold an election before 2020 yesterday. In a surprise announcement on the steps of Downing Street, she urged voters to hand her a Brexit mandate before formal talks with Brussels began.

The pound surged to a six-month high as the markets bet on the prospect of a softer Brexit, with experts claiming that a bigger majority would leave Mrs May less exposed to “right-wing factions” within her party.

Today the prime minister will begin a seven-week campaign with an attack on Mr Corbyn’s leadership at prime minister’s questions before a Commons vote that clears the way for a general election on June 8. Britain’s third national poll in as many years looks certain to bring a political realignment.

Mr Corbyn welcomed the election announcement, saying: “Labour will be offering the country an effective alternative to a government that has failed to rebuild the economy, delivered falling living standards and damaging cuts to our schools and NHS.” The party looked forward to showing how it would “stand up for the people of Britain”.

Mrs May pitched her message to disaffected Labour voters, however, as she asked them to increase her working majority of 17. “Our opponents believe, because the government’s majority is so small, that our resolve will weaken and that they can force us to change,” she said.

This morning she told the BBC’s Today programme that even if she succeeds in increasing her majority in the Commons there would still be room to debate the terms of Brexit.

She denied that she wanted to suffocate dissent, adding: “As we go through parliament with Brexit we will of course be challenged and there will of course be debate. But what the British people want is for the government to deliver on the vote they gave to leave the European Union, and that’s what we will do.”

Without a snap election, Mrs May said that “political game-playing” in Westminster would continue, with EU negotiations reaching their “most difficult stage” in the run-up to the previously scheduled 2020 vote. “Division in Westminster will risk our ability to make a success of Brexit, and it will cause damaging uncertainty and instability to the country,” she said, adding that the period before formal talks offered a “one-off” chance to settle the country’s future.

Mrs May will receive an early boost today, with YouGov polling data for The Times suggesting that the swing from Labour to Tory was bigger in Labour seats that voted to Leave than the national average. Polls this month put the Conservatives on an aggregate of 43 per cent, Labour on 24, the Liberal Democrats on 11 and Ukip on 10. Assuming a uniform national swing, the Tories would have 382 seats, Labour 179, the Lib Dems 10, the SNP 56, and others 23, giving the government a majority of 114, according to Anthony Wells, of YouGov.

John Curtice, another elections expert, said that Mrs May’s ambition for a three-figure majority would melt if the Tories’ polling lead were cut to single figures. “In 2015 a seven-point lead over Labour was only enough to get a majority of 12,” he said.

A surge of 5,000 in Lib Dem membership yesterday fuelled jitters among Tory MPs in seats that voted Remain last year. Tim Farron, the party’s leader, urged voters to take the chance to “change the direction of your country”.

Tony Blair called on the public to put Britain’s relationship with the EU before party affiliation. He appeared to back the election, saying that holding off would have been an “extraordinary act of political self-denial” by Mrs May.

The prime minister was greeted by cheering and clapping Tory MPs in a meeting of the 1922 committee, as well as chants of “five more years”.

Launching the SNP’s general election campaign in Westminster today, Nicola Sturgeon said she was “fighting to win as many seats and as many votes as possible”.

The first minister said she would form an anti-Tory coalition with Labour and the Liberal Democrats if the June result made it possible. “I don’t want to see a Tory government,” she said. “If there was a parliamentary arithmetic which leant itself to keeping the Tories out of government I would want to see that happen. Are you asking me do I think that’s likely? My answer is that’s not what I’m looking at right now.

“My objective in this election is not to worry about coalitions with anybody, but to make sure that the SNP is in the strongest possible position to stand up and protect Scotland’s interests.”