Labour's top candidate in Scotland for next week’s European elections and more than half of the party's Scottish MPs have said staying in the EU is "non-negotiable" in a direct challenge to their leadership at Westminster.
In an open letter to voters, David Martin MEP, who leads the Labour list in Scotland, said a 'confirmatory' referendum on any Brexit deal agreed by Westminster was the only way to deliver on Jeremy Corbyn's aims of a fairer, greener UK and Europe.
And it calls on Labour to embrace the manifesto put out by the European socialist group, suggesting that MEPs elected on 23 May could sit in the European Parliament "potentially for the next five years."
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The MPs Ian Murray, Martin Whitfield, Ged Killen and shadow Scotland minister Paul Sweeney have also signed the letter, as well as fellow European election candidate Callum O'Dwyer.
It comes amid uncertainty over Labour's stance on cross-party Brexit talks with Theresa May's government, which are set to continue, and whether the opposition will follow through on demands from members and scores of MPs to seek a second EU referendum.
While Labour's deputy leader Tom Watson has said Labour is a "remain and reform" party, senior figures on Mr Corbyn's front bench have continued to insist Labour will deliver Brexit.
"As elected politicians we firmly believe that it is in the UK and Scotland’s best interest to continue to play an integral role at the heart of the European union. That is why the European Elections are so important," the letter states.
"There is no doubt that the overriding issue at the moment is Brexit, but we must also be mindful as voters that we are electing our representatives in the European Parliament potentially for the next five years.
"We are very proud of the manifesto produced by the European wide Party of European Socialist (PES) grouping of which the Scottish and UK Labour Party MEPs are the only members from the UK affiliated."
The letter goes on to highlight Europe-wide proposals to force multinationals like Google, Amazon and Facebook to pay their fair share in tax, close the gender pay gap, reduce greenhouse gas emissions, and protect consumer rights.
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"Of course, this socialist group manifesto can only be delivered and benefit Scottish citizens by both electing socialist group MEPs through the Labour Party AND remaining in the EU," the letter states.
"We believe that remaining in the EU is non-negotiable and we will help deliver that through a public confirmatory vote on any deal agreed by parliament, to give you a final say. That is putting the national interest first and foremost.
"We believe in a public vote to allow revocation, renewal and reform. Let’s elect MEPs who believe in a better future leading and not leaving the EU."