Jeremy Corbyn has been told his party may never be forgiven if it works with the Conservatives to bring in a “hard-right Brexit”.
Scottish Green co-convener Patrick Harvie issued the warning, as he told party activists to “get into campaign mode” in case the UK takes part in European Parliament elections next month.
With Theresa May seeking an extension of the Brexit process until the end of June, European leaders may insist that the UK elects MEPs in the May 23 ballot.
Without an extension Britain is facing the prospect of leaving the European Union without a deal on Friday.
Talks were taking place between the Conservatives and Labour in a bid to find a way through the deadlock, but these have now stalled.
And while Downing Street has offered further talks over the weekend, Labour insisted the Prime Minister had to come forward with “genuine changes”.
But Mr Harvie said that Labour “even now seems willing to strike a deal with the government they should be opposing to drag us out of Europe”.
Speaking at the Scottish Green conference in Edinburgh, he said: “We are now seeing the threat of a Labour/Tory deal to finally complete Brexit and take us out of Europe.”
With Labour and the Conservatives having worked together before in the run-up to the Scottish independence referendum in 2014, Mr Harvie added: “Better Together are together again. I think we need to give a very clear message to the Labour Party – do not take up this absurd offer of sharing the blame for Brexit.
“Do not trust a word you hear from a Tory Party that doesn’t even know who its next leader will be, and do not expect to be forgiven if you facilitate this hard-right Brexit project.
“A People’s Vote or a simple revoke, these are the only two options left.”
He recalled that Scotland “voted overwhelmingly to remain in the European Union” in 2016 and said that since then “we’ve been cut out and ignored by the UK Government at every turn”.
He hit out at the Conservatives at Westminster for trying to avoid having European elections.
Mr Harvie said: “This isn’t just a fear of democracy, the same fear that is leading them to try to refuse giving the public their final say on Brexit itself, it is also an enormous missed opportunity for a debate about the issues that matter.”
And he told his party: “Let’s welcome the opportunity to get out there on the doorstep, get into campaign mode over the coming weeks ahead and elect someone who can make a case in Europe for the Green new deal that Scotland needs. A Green MEP that will agitate for an urgent and radical shift away from the oil and gas industry into clean energy.”
The Green MSP also insisted people in Scotland “must and will have the chance to take our future into our own hands again” in a second independence referendum.
He hit out at the UK Government for blocking Brexit legislation passed by the Scottish Parliament after MSPs there refused to give consent to UK Withdrawal Act.
This, Mr Harvie insisted, was the “single greatest sign of contempt ever shown by the UK political system since the Scottish Parliament’s creation”.
He stated: “The UK Government didn’t like what we were doing so they triggered a court case after we passed the Bill to prevent it getting Royal Assent, then they legislated retrospectively to limit the powers of Holyrood on devolved issues, making it impossible for the Bill to come into force.
“This is far more than just a procedural battle between two governments – it is a clear sign that what we have now 20 years after the people of Scotland finally achieved their own parliamentary democracy is the clear knowledge that whatever laws we pass on devolved areas of authority the UK is not only able but fully willing to retrospectively bind our hands and block the laws we make when they disagree.”