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Theresa May is dealt a blow today as a new poll reveals support for Jeremy Corbyn surging in London.

For the first time, more voters in the capital say they think Labour’s leader would make a better Prime Minister than Mrs May.

The Tories’ hopes of gaining seats in London are dashed by a huge 17-point lead opening up for Labour. Instead, they could lose seats. Mrs May came out fighting with a speech in the North-East at lunchtime. “I have the determination and I have the plan,” she said, calling the election “the most important this country has faced in my lifetime”.

She added: “If we get Brexit right, I am confident the future will be bright.”

The current standing of the parties reflects how London voted in 1997 when Tony Blair won his landslide first victory, according to the YouGov poll of 1,000 Londoners produced for Queen Mary University of London. Labour is on 50 per cent, up from 41 per cent a month ago. The Tories are on 33 per cent, down from 36 in a month. In March the parties were just three points apart, at 37/34.

Asked who would make the best Prime Minister, 37 per cent picked Mr Corbyn and 34 per cent Mrs May. A survey taken just after the manifesto launches last month had Mrs May ahead by 38 to 32.

The findings are further evidence that the Tory campaign lost its way after the row over the so-called dementia tax.

Boris Johnson, the Foreign Secretary, said his party was “fighting for every vote” as a separate national survey this morning reduced the Tory lead to three points.

Mrs May hoped to get back on track today with her speech focusing on the “great national mission” of Brexit, her strongest issue. She urged voters in Teesside: “The brighter future we want for our country will not just happen.”

In London, Mr Corbyn was giving a rival vision for Brexit, saying Labour would protect the economy, jobs and living standards in negotiations with Brussels.

Professor Philip Cowley, director of Queen Mary University’s Mile End Institute, said: “This wasn’t part of the Conservative script for the election.

“They didn’t expect to be looking at potential losses in London. It’s also striking how the smaller parties are struggling in the capital. The combined Labour and Conservative vote in London is now higher than at any time since 1979.”

Mr Johnson was deployed this morning to rally Tory supporters. “For me it is great that this is tightly fought,” he said. “It is great that people think that this is a very hotly contested election, because it is. We are fighting for every vote, because the future of our country is at stake.”

The former London mayor said Home Secretary Amber Rudd, whose father died on Monday, had given a “heroic” performance in last night’s TV debate on BBC1. Mr Johnson accused the BBC of bias, saying it had been “the most Left-wing studio audience the BBC has ever brought together”.

Work and Pensions Secretary Damian Green said that unemployment would be “far” higher under Mr Corbyn’s plans for increased taxes and benefits.

Surprisingly, given the fierce Conservative advertising campaign on social media to highlight Mr Corbyn’s record of opposing anti-terrorist laws and his links with Sinn Fein and Hamas, he and Mrs May are virtually neck-and-neck when Londoners were asked who they trust to keep the country safe from extremists. Mrs May is trusted by 42 per cent and distrusted by 46. The Labour leader, who has argued that fewer foreign wars would reduce the terrorism threat, is trusted by 41 and distrusted by 47.

The fine detail of the survey reveals that Mr Corbyn is more popular with women than men. Mrs May is preferred by male voters.

Mr Corbyn is strikingly unpopular with older voters, with over-65s dividing three-to-one for Mrs May and the Conservatives. But he has a clear lead among younger people and Remain voters.