Labour and the Liberal Democrats will try to reboot their campaigns in the final fortnight of the election after a new poll showed them being hammered in Leave constituencies.

Jeremy Corbyn and Jo Swinson are being forced to change tack by the failure of their initial plans to win over voters, and advances by Boris Johnson’s Conservatives.

The mid-campaign shift – often a sign of weakness in elections – comes as all the parties digest the findings of a 100,000-voter YouGov poll pointing to a 68-seat Tory majority.

The survey method, which predicted a hung Parliament in 2017, found 43 per cent of voters across the UK would back the Tories in an election tomorrow, compared to 32% for Labour, 14% for Labour and 3% for the Brexit party.

It would translate into 359 seats for the Tories, 211 for Labour, 43 for the SNP, 13 for the LibDems, four for Plaid Cymru and one Green.

The bulk of Tory gains would be in the north of England and Midlands, breaking through the so-called “red wall” that Labour needs to preserve to have any chance of power.

In Scotland, the SNP was predicted to gain eight seats by picking up five of Labour’s seven, two of the Tories’ 13, and one of the four held by the LibDems.

The BBC reported Labour was moving to a more defensive campaign after underestimating the willingness of Leave voters to switch from Labour to Tory and over-estimating the threat from the LibDems.

Instead of attacking Tory seats, Labour will focus on shoring up support in vulnerable areas such as the Midlands to persuade Leavers of Labour’s plan for an EU referendum in which Mr Corbyn would remain neutral as an “honest broker”.

It means a lower profile for Labour frontbenchers closely associated with Remain, such as Sir Keir Starmer and Emily Thornberry, and a greater role for Leavers such as party chairman Ian Lavery and the more Brexit-friendly Richard Burgon, Laura Pidcock and Jon Trickett.

There will also be a big push on the risk of returning Mr Johnson as PM for five years in the hope Labour can squeeze the LibDem vote.

Campaigning in Southampton, Mr Corbyn did not respond directly to questions on whether he had changed Labour’s election strategy. He stressed his plan for securing a “credible” Brexit deal before putting it to the country in a referendum within six months to bring the UK together.

“That is a message I will take out all over the country and our campaign is in every part of the country,” he said.

“Do you know what, I say the same thing in every place I go. I don’t have one message for one group and another message for another group.”

However his closest ally, Unite general secretary Len McCluskey, said the party did have to explain to working class Leave communities why they would be better off under Mr Corbyn than Mr Johnson.

He said: “What Labour needs to do is try and explain that it’s only Labour that offers a credible way of actually dealing with Brexit and dealing with the type of Britain that we want in the future, what type of country do we want. It’s Labour that has to get across that message.”

Brexit Party leader Nigel Farage said Labour’s apparent campaign shift was “almost an acceptance that they have lost the election”. Speaking at a golf driving range in Hull, he told supporters: “I think the truth of it is, they realise they are now fighting a defensive battle, they know they can’t win the election. “This is now a defensive strategy to try to hold the seats that they have held for decades.”

Ms Swinson, who started the election touting herself as the Prime Minister who would revoke Article 50, also changed direction, focusing on attacking Mr Johnson instead.

She admitted it was now “pretty unlikely” she would be in Downing Street on December 13 and urged people to vote tactically for her party to deny the Tories a majority.

In a sustained attack on Mr Johnson, she said he was “not fit to be Prime Minister” after “lying to the Queen” over his unlawful attempt to prorogue parliament.

She said: “Boris Johnson only cares about Boris Johnson. He will do whatever it takes, sacrifice whatever or whoever is needed to get what he wants. This is a man who decided which side to support in the EU referendum by game-playing what would be most likely to get him the keys to Number 10.

“His life has been about becoming Prime Minister. Not out of some burning desire to make people’s lives better, but out of some sense of Etonian entitlement, because it’s what people like him get to do. “Boris Johnson doesn’t care about you and your family.”

She highlighted an endorsement by ex-English Defence League founder Tommy Robinson as evidence Mr Johnson had far-right support.

She said: “Honestly, such an endorsement would shame any decent person but Boris Johnson has no shame when it comes to the language he uses about race.”

She tried to put a brave face on the YouGov poll showing a single LibDem gain, and the party to pick up in its key target area, the Leavesupporting South West of England.

She said: “What this poll shows is there are 134 seats where either the Liberal Democrats are in first or second place as things stand. That shows a huge amount to play for.”

A new Ipsos Mori poll for STV also showed the SNP doing better than in the 2017 election.

It put support for Nicola Sturgeon’s party on 44 per cent, the Tories on 26%, Labour on 16% and the LibDems on 11%.

A seat projection by Electoral Calculus said the figures would translate into 48 MPs for the SNP (up 13, six for the Tories (down seven), the LibDems unchanged on four, and Labour losing six of their seven MPs.

However the SNP’s decision to suspend its candidate in Kirkcaldy & Cowdenbeath for alleged anti-Semitism last night could see Labour’s Lesley Laird hang on to her 259-vote majority in a seat the Nationalists had been expected to win easily.

The Scottish Greens are now asking pro-independence supporters to back them instead.

The survey of 1,046 Scots also found a 50-50 split on independence, although most Scots were opposed to another referendum on Ms Sturgeon’s timetable of late 2020.

Despite the polls, Scottish LibDem leader WiIlie Rennie will today launch his party’s manifesto in Edinburgh with a promise to stop Brexit, stop independence, and “build a brighter future”.

Mr Rennie said: “If we make the constitutional chaos stop we can focus on the issues that really matter to people on a day to day basis. We’re putting forward a plan that is radical, credible and progressive.”

Mr Johnson refused to say whether he would stay on as Prime Minister if he loses his Uxbridge seat but wins a majority overall in the country.

He said: “We’ll be fighting for every seat all over the country.” Pressed again, he said: “I’m hoping very much to win again in Uxbridge and South Ruislip and fighting hard for every vote.”