In a last-ditch effort to stop a hard Brexit, campaigners for a Final Say referendum are calling on Liberal Democrat and Labour candidates in 20 key general election seats to cease campaigning and announce they are voting for Remain-backing rivals who stand a better chance of denying Boris Johnson a majority.

The eve-of-poll plea came as new polling suggested that in at least one constituency – Stockton South – a few thousands votes for the third-placed Liberal Democrats could make the difference between Labour and Conservative victory on Thursday.

The call was backed by the prominent parliamentary campaigner for a referendum Peter Kyle, who is fighting to retain his seat in Hove for Labour but urged the party’s supporters in areas where they stand no chance of winning to lend their votes to other anti-hard Brexit candidates better-placed to beat Tories.

The move followed a call from Nigel Farage for Leave supporters to vote tactically for his Brexit Party to provide it with a “bridgehead” in parliament from which to put pressure on Tories for the hardest possible EU withdrawal.

The Vote for a Final Say (VFS) campaign issued a list of 15 Lib Dem candidates and five from Labour who it said should “get out of the way” of referendum-backing rivals.They urged supporters of these candidates to put pressure on them via social media to call off their campaigns.

Top 10 marginal seats to watch at the general election Show all 10 1 /10 Top 10 marginal seats to watch at the general election Top 10 marginal seats to watch at the general election North East Fife Currently held by the SNP with a majority of 2 Getty Top 10 marginal seats to watch at the general election Kensington Currently held by Labour with a majority of 20 Getty Top 10 marginal seats to watch at the general election Perth and North Perthshire Currently held by the SNP with a majority of 21 Getty Top 10 marginal seats to watch at the general election Dudley North Currently held by independent, formerly Labour, MP Ian Austin with a majority of 22 LivingInMediocrity Top 10 marginal seats to watch at the general election Newcastle-under-Lyme Currently held by Labour with a majority of 30 Derek Harper Top 10 marginal seats to watch at the general election Southampton Itchen Currently held by the Conservatives with a majority of 31 Rob Candish Top 10 marginal seats to watch at the general election Richmond Park Currently held by the Conservatives with a majority of 45 Robin Webster Top 10 marginal seats to watch at the general election Crewe and Nantwich Currently held by Labour with a majority of 48 Jaggery Top 10 marginal seats to watch at the general election Glasgow South West Currently held by the SNP with a majority of 60 Alec MacKinnon Top 10 marginal seats to watch at the general election Glasgow East Currently held by the SNP with a majority of 75 Christine Johnstone

At this late stage in the campaign, it is not possible for candidates’ names to be removed from the ballot paper or for voters to be stopped from marking their X by their names. But VFS believes that a public endorsement could be enough to get big chunks of support to switch.

Among seats on the list were Liberal Democrat targets Cheadle, Cheltenham, Winchester, South Cambridgeshire and St Albans, where Jo Swinson’s party could be assured of victory if Labour voters lent their ballots to the Lib Dem candidate.

Constituencies where the group recommended Lib Dem candidates ask their supporters to vote Labour included Brexit figurehead Iain Duncan Smith’s Chingford and Woodford Green, where the former Tory leader is believed to be vulnerable to a challenge from Faiza Shaheen, and Putney, where Labour are hoping to snatch the onetime seat of ex-minister Justine Greening.

In Stockton South, the campaign urged Lib Dem Brendan Devlin to endorse his Labour rival Paul Williams in the hope of helping him hold on to the ultra-marginal seat which he won by just 888 votes in 2017.

Polling by Survation suggested that Tory Matt Vickers is currently on course to win the North-East England seat by a margin of 46 per cent to 43 per cent, with Mr Devlin trailing in fourth, behind the Brexit Party, on just 3.4 per cent. But the poll suggests that if the Lib Dem candidate urged supporters to back Dr Williams, he could hold the seat by a margin of 48-43.

Peter Kyle is fighting to hold on to Hove for Labour (PA Wire/PA Images)

Liberal Democrats have stood aside for Green and Plaid Cymru candidates as part of the Unite to Remain alliance, and are not putting up candidates against former Tory europhiles Dominic Grieve and Anna Soubry. But leader Jo Swinson has set her face firmly against any deal with Jeremy Corbyn’s Labour.

Mr Devlin last month appeared to accept in a tweet that both he and Dr Williams were options for “pro-European” voters. But he later insisted: “I have not endorsed the Labour Party and I never will… The national Labour Party is extreme, inept and losing friends by the day.”

Dr Williams said: “If Brendan Devlin asked people to vote for me instead of the Liberal Democrats here in Stockton South then it would not only be a kind and typically principled act, but it would also stop us going back to having a Tory MP. If I were re-elected I would fight for a fairer voting system that makes every vote count, so that this type of compromise would never have to happen again.”

Elsewhere, VFS are urging Lib Dem candidates to stand aside in constituencies won by Labour in 2017 where Mr Corbyn’s party are hoping to avoid losses to Tories.

They include Canterbury, where original Lib Dem choice Tim Walker stood down in favour of Labour’s pro-referendum Rosie Duffield, only for Ms Swinson to impose another candidate.

Mr Walker said: “I beg other Lib Dem and Labour candidates in identical situations to my own to now look to their consciences and abandon their disreputable campaigns.

Rosie Duffield won Canterbury for Labour by just 187 votes in 2017 (Reuters)

“I know of dozens who know deep down what they’re doing is wrong. They should announce they are voting for the Labour or Lib Dem candidate who can win – and urge their supporters to do likewise.

“I beg them to be their own men and their own women at this time of reckoning. To face up to reality before it is too late.”

Other seats where VFS are urging Lib Dems to step aside are Wakefield, Warrington South, Weaver Vale, Sedgefield, Gower, Eltham, Bristol North-West, Croydon Central, Enfield Southgate, Wirral West and Warwick and Leamington – all Labour in 2017 and all vulnerable to Tories.

Mr Kyle, a prominent figure in the parliamentary battle for a second referendum, said: “This election is like no other so we’re all having to think differently to stop Boris Johnson’s brutal Brexit.

“If you believe in your heart that Labour can’t win in your area but an anti-Brexit party can, then just this once put your Labour values into action in a different way by lending your vote to a ‘stop Boris Johnson’ candidate. Our country cannot take another five years of Tory government, let’s do whatever it takes to stop it happening.”

Former Lib Dem candidate Rob Castell, who stood down in Beaconsfield to help rebel former attorney general Dominic Grieve retain his seat, said: “This election is a choice between Boris Johnson getting the majority he needs to force through his destructive Brexit or an alliance representing the majority coming together to deliver a Final Say referendum.

“Third-placed candidates risk muddying the waters on this choice.”

In a separate move, anti-Brexit campaigner Femi Oluwole of Our Future Our Choice released his own list of third-placed candidates he believes should cease their campaigns to help block a Johnson majority.

While several of them coincided with the VFS list, OFOC also called for Labour candidates to stand aside in Lewes, Norfolk North, Guildford, Carshalton and Wallington, Esher and Walton, and Wokingham and Lib Dems in Harrow East, Bishop Auckland, Ipswich, Bedford, Workington and Lincoln.