A Liberal Democrat candidate in the marginal seat of Stockton South has effectively endorsed his Labour rival, urging supporters to “vote tactically” to defeat the Conservatives.

In a string of tweets, Brendan Devlin said he could not endorse Jeremy Corbyn or the Labour Party nationally, but regarded the party’s local candidate Paul Williams as “an honest, sincere man”.

Acknowledging the seat was “a Lab/Con battle”, he urged his supporters to “vote tactically for someone who cares about the NHS and social care” – effectively pointing them to put their cross next to Dr Williams’ name.

Brendan Devlin’s move came after campaigners for a Final Say referendum on Brexit issued a call for 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.

Mr Devlin was top of the Vote for a Final Say (VFS) list, as polls suggested his small band of supporters could make the difference between Labour or Conservatives seizing the seat.

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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.

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.

And Mr Devlin last month insisted he would never endorse the national Labour party, describing it as “extreme, inept and losing friends by the day”.

But on the day before polling, Mr Devlin released a statement effectively throwing his weight behind Dr Williams’ battle to retain the seat which he won by just 888 votes over Tories in 2017.

The Lib Dem candidate said: “Polling shows that Stockton South is a Lab/Cons battle. I cannot and do not endorse either Party or their leadership.

“But I have met Paul; he’s an honest, sincere man, and if I needed help, I know he’s reliable and committed to social justice. And he turns up to hustings.

“Liberal Democrats are a party of reason motivated by human dignity and concern for our country; in these final moments of this election, I urge all voters to choose wisely with those thoughts in their minds.

“To be crystal clear: vote tactically for someone who cares about the NHS and social care. Those are important in Stockton South.”

Dr Williams responded: “Thankyou for the personal endorsement Brendan Devlin. You’ve been able to put party and personal interest to one side in favour of the national interest. I’ve grown to know you over the last five to six weeks as a truly kind and honourable man.”

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.

VFS is calling on 14 more Lib Dem candidates and five from Labour to “get out of the way” of the drive to block a hard Brexit by halting their campaigns and urging supporters to vote for rivals who are better placed to defeat Tories.

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 threw their ballots behind the Lib Dem candidate.

Constituencies where the group recommended Lib Dem candidates to stand aside 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.

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.

“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.”