Nigel Farage is under growing pressure to stand down his candidates from next month's election.
The Brexit Party leader was warned Britain could end up with a 'shabby coalition of socialists, Lib Dems, Scottish and Welsh nationalists' if he contests seats the Tories are trying to win.
A string of Mr Farage's candidates are leading calls for him to pull back.
Nigel Farage (pictured in Pontypool in south Wales on Friday) is under growing pressure to stand down his candidates from next month's election
Writing in this newspaper, the former Brexit Party hopeful in the battleground seat of Workington begs colleagues to copy his decision to withdraw.
Philip Walling said not doing so could help deny Boris Johnson a majority and increase the 'hideous' prospect of Labour's Jeremy Corbyn in No 10.
He says: 'I plead with every other Brexit candidate to do what I have done: examine your conscience in the cold light of reason. And if you think you risk splitting the Tory vote, and so damaging the best chance of Brexit that Britain has got, then for God's sake – stand down.'
A growing number of Mr Farage's associates and donors – including his ally Arron Banks – have also launched a revolt against his refusal to call an election truce with the Tories.
A major YouGov poll of 11,500 voters shows the Brexit Party presents a serious threat to Mr Johnson's hopes.
Although Labour support is collapsing across the country, the Brexit Party is polling as high as 19 per cent in some regions.
Analysis of polling figures last week found that the Brexit Party could make a crucial difference in almost 90 Tory target seats. In 38 of those, Mr Johnson could snatch victory from Labour if 70 per cent of those planning to vote for Mr Farage switched to the Conservatives.
Yesterday's YouGov poll reinforced those findings, showing that the Brexit Party is recording high poll ratings in a string of regions.
In the North East, it is on 19 per cent, up from the 4 per cent that Ukip recorded at the 2017 snap general election.
The predicted Tory vote share has fallen in every region since 2017 but the poll is most disastrous for Labour, with its support collapsing across the country.
In Yorkshire and Humber, the predicted Labour vote share is now just 29 per cent, compared with 49 per cent in 2017. Nigel Evans, the Tory candidate for Ribble Valley in Lancashire, said: 'The danger in the North West and North East will be where a Brexit candidate steals sufficient votes to allow the Remain candidate to win.
'A number of Brexit candidates have already decided they know the strategy is not working, they know the dangers, and they don't want to lose Brexit.
'Yet that is exactly what Nigel Farage's strategy could do.'
Yesterday, another former Brexit Party candidate – Peter Udale, who was due to stand in the Cotswolds – quit and urged voters in his area to back the Tories instead.
He said: 'By far the biggest threat that currently faces this country is a government led by Jeremy Corbyn. If he got into No 10, he would undermine our economy, our defence and our union.
'I therefore believe it is fundamentally wrong for the Brexit Party, which is strongly patriotic and has a deep allegiance to our union, to stand candidates in constituencies where the Tories have a chance of winning.'
A growing number of Mr Farage's (pictured in Newport, Wales, on Friday) associates and donors – including his ally Arron Banks – have also launched a revolt against his refusal to call an election truce with the Tories
Professor Matthew Goodwin from the University of Kent said Mr Farage risked 'damaging his legacy' by taking votes from the Tories.
Speaking on the Daily Telegraph's Brexit Podcast, he said: 'I don't think they will win any MPs ... Nigel Farage clearly has to ask himself a question at this point – does he want to go down in British history as the most influential politician not elected to Westminster who effectively brought about Brexit?
'Or does he potentially want to go down in history as the guy who attracted 5 to 7 per cent at the election and cost Boris Johnson and the Conservatives an election victory and by extension brought down Brexit. This is the fundamental choice facing both Farage and the Brexit Party.'
Multi-millionaire Mr Banks, who campaigned with Mr Farage under the Leave.EU banner during the 2016 referendum, said: 'Like everything in life, what is the point of doing something if you can't win?
'He risks splitting the vote and letting a Lib Dem through the middle to win – a party which wants to cancel Brexit altogether.'
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