Theresa May has been warned that Brexiteer anger will “explode” if she strikes a Brexit pact with Jeremy Corbyn to keep the UK closely tied to the European Union.

Brexit Party leader Nigel Farage claimed the UK’s democracy was under threat as he sought to build momentum for his movement ahead of the European elections.

His comments came after Foreign Secretary Jeremy Hunt warned that Mrs May could struggle to hang on to power if she cannot get her Brexit deal through Parliament before the May 23 European poll.

Mr Hunt said the “total focus” of ministers was to ensure the country did not have to vote in the elections to the European Parliament on May 23.

Speaking during a visit to Japan, he acknowledged that the Government would be facing a “very serious situation” if it failed to do so.

But Mr Farage, speaking at a rally in Shoreham-by-Sea, said even a Brexit deal with Labour which allowed the elections to be cancelled would not save her from a Eurosceptic backlash.

He said: “The only way they could be stopped is if Mrs May signed up to a deal with Mr Corbyn which kept us stuck, permanently, in a customs union and under single market rules.

“A message to you Prime Minister: If you think by doing that deal and cancelling the European elections that the threat of Farage and the Brexit Party will go away, you are in for a rude shock.

“If you betray us that much, this will explode.”

Nigel Farage addressing the Brexit Party rally (Gareth Fuller/PA)
Nigel Farage addressing the Brexit Party rally (Gareth Fuller/PA)

He told his supporters: “What we are now fighting for is much, much bigger than Brexit.

“What we are now fighting for is for the survival of the very principle of democracy in this country.”

Mr Farage’s new party was boosted by the defections of three MEPs, who followed him in shifting over from Ukip.

Deputy chairman and East Midlands MEP Margot Parker, West Midlands MEP Jill Seymour and Yorkshire and North Lincolnshire MEP Jane Collins announced their resignations on Monday.

Former British Chambers of Commerce director general John Longworth has also confirmed he will stand for the party in the European elections.

On the other side of the Brexit divide, the pro-EU centrist party Renew has given its backing to The Independent Group as the breakaway former Labour and Tory MPs seek to form their new party to contest the European elections.

Many Tory MPs are furious that the elections may have to take place at all following the delay to Brexit.

Mr Hunt acknowledged that the party could be heading for a “disastrous” showing at the polls if the country was required to vote.

“In terms of polling it certainly looks that way,” he told BBC Radio 4’s Today programme.

Mr Hunt said it would be “highly, extremely, very, very challenging” for Mrs May if she was not able to get her deal through.

“That would be a very serious situation – I don’t pretend otherwise – but we aren’t at that point,” he said.

He said talks with Labour – aimed at finding a cross-party consensus on the way forward – were proving “more constructive” than many at Westminster had expected.

A series of working groups are being set up within the Brexit talks with Labour to look at specific issues, Downing Street said.

Business Secretary Greg Clark and his Labour shadow Rebecca Long-Bailey will look at services and consumer and workers’ rights, Environment Secretary Michael Gove and his counterpart Sue Hayman will consider environmental protections, and Brexit Secretary Stephen Barclay and Labour’s Sir Keir Starmer will look at security.

With Parliament in recess, and Mrs May on a walking holiday in North Wales, no imminent breakthrough is expected.

Prime Minister Theresa May (Stefan Rousseau/PA)
Theresa May became Conservative leader and Prime Minister in 2016 (Stefan Rousseau/PA)

Meanwhile, the Prime Minister’s potential successors jostled for position in the Tory leadership race, with Home Secretary Sajid Javid making a high-profile speech on tackling crime which focused heavily on his personal history.

Chancellor Philip Hammond mocked prominent Tory Brexiteers for engaging in a “suicide pact” during failed bids to beat Theresa May to the Tory leadership.

Mr Hammond used a speech in the US on Friday to say Mr Gove and former foreign secretary Boris Johnson had formed an “unintended suicide pact” in the 2016 leadership contest, the Daily Telegraph said.

How a Conservative leadership election works graphic
(PA Graphics)

The Chancellor said that Commons Leader Andrea Leadsom had effectively “knifed herself” during the race to become Prime Minister, according to the newspaper.

Mr Hammond said in a speech at the British embassy in Washington DC the Tories have the “joy of a leadership contest ahead”.