More than 10,000 Labour members have signed up within hours to a new declaration calling on the party to lead the campaign for the UK to remain in the European Union.

The Proudly British Proudly European declaration was posted online by deputy leader Tom Watson on Thursday evening as pressure mounted on Jeremy Corbyn to throw the party’s weight behind a Final Say referendum and a Remain vote.

Issued as polls showed the party slumping into fourth place with 18 per cent support, the declaration warns that Labour will “never” win an election as a party of Brexit. And it says that a people’s vote can only be delivered with Labour “leading the way”. Senior Labour sources have indicated that a statement on the “evolution” of the party’s thinking on Brexit can be expected within weeks.

But members of Corbyn’s inner circle are believed to be resisting full-throated support for continued EU membership, while the leader himself has been discussing the example of Harold Wilson, who allowed his MPs to campaign on both sides in the 1975 referendum while himself remaining aloof from the debate.

The new declaration, carried on the website labourremain.org , states that Labour “must lead the campaign to Remain”. In a swipe at supporters of “Lexit” – left Brexit – who argue EU withdrawal will free a Labour government to provide state aid to key industries and embark on a programme of nationalisation, the declaration dismisses the idea of “socialism in one country” as “outdated”.

And it states: “Being European is part of our national identity and the EU is its institutional expression. Just as being British makes us more than solely English, Scottish, Welsh or Irish, so being European makes us more than just British, but not a jot less.

“As democratic socialists, in particular, we embrace an entity in which our values are so deeply embedded. They’re institutions we helped to shape, which lead the world on workers’ rights and climate change, fair trade and balanced regulation.

“Like Labour, it’s in the nature of the European Union that it stands up for the weak against the strong, protecting workers from exploitation... protecting markets and consumers from huge trans-continental oligarchies.”

‘Labour values are European values,’ Tom Watson says (Reuters)

The declaration warns: “Without power we can achieve nothing. As a party of Brexit, we will never win again, and that is an abrogation too far.

“As the party of Remain, we will not take every voter with us, but it’s the only way that Labour can win and the only way to keep our country together.

“Because we love Britain and because we believe in Labour, we will fight, fight and fight again to keep Britain in the EU. The next step, which only Labour can deliver, needs to be a public vote on the Brexit fiasco, with Labour leading the way for Remain.”

Signatories to the declaration trigger an automatic email to their local Labour MP or councillors, informing them they have signed and urging them to do the same.

By 10am on its first morning, the declaration had attracted more than 10,000 signatures from Labour members and supporters.

MPs who have already signalled their support include Stephen Doughty, Maria Eagle, Wes Streeting, Neil Coyle, Ruth Cadbury and Ben Bradshaw.