Nicola Sturgeon has warned Labour MPs that it would be the “end” for the party in Scotland if they help pass Boris Johnson’s Brexit bill.
Scotland’s first minister was speaking ahead of a special Saturday sitting of Parliament – the first in 37 years.
The prime minister is trying secure the 320 votes the UK government needs to ensure a parliamentary majority.
Ms Sturgeon believes some Labour MPs will rebel against Jeremy Corbyn and back the deal.
In a briefing with reporters at Westminster she said: “I think it would be the end for Labour in Scotland if they turn out to be the handmaidens of Boris Johnson’s Brexit.”
Labour’s seven Scottish MP are expected to vote against the deal.
Meanwhile, Scottish Secretary Alister Jack told BBC Scotland that he believes any Conservatives who do not back the bill should have the whip taken away from them.
Without the backing of the DUP, which has said it cannot support the revised bill, the vote is likely to be extremely tight.
Speaker John Bercow may also select a number of amendments for debate, including one from former Tory MP Oliver Letwin, which has cross-party support.
Ian Blackford, the SNP’s Westminster leader, has also tabled an amendment to the motion, rejecting the new Brexit deal.
He also calls for an extension until at least 31 January 2020, allowing for an early election.
The first minister said final decisions on how the SNP plans to vote will not be taken until Saturday morning.
Speaking on Friday, she added: “My growing suspicion today is that Labour will allow Boris Johnson to get his deal over the line tomorrow.”
Although Labour’s official position is to oppose the deal the first minister fears “nods and winks” are being given to party rebels.
Ms Sturgeon also believes the choice was not simply between the prime minister’s deal and no deal.
She said: “The alternative to this deal tomorrow is the Benn Act, which would require an extension request. That’s the law of the land.
“So anybody who says that it’s a choice tomorrow between this deal and no deal is frankly not being straight with people.”
Ms Sturgeon said her party will vote against the deal but would be sympathetic to voting for a second referendum.
Meanwhile, in an interview with BBC Scotland’s The Nine, Alister Jack admitted the result will be “very close”.
The Scottish Secretary also confirmed that there would be no border checks at Cairnryan under the terms of the deal – they would be at Larne and Belfast in Northern Ireland.
He added that he would like to see any Tory rebels who fail to back the deal have the whip taken away from them.
Twenty-one MPs were expelled from the parliamentary Conservative party after they rebelled against the government in a bid to prevent a no-deal Brexit in September.
“It’s a decision for the prime minister and the chief whip but I think we should treat everyone equally and what’s good for the goose is good for the gander,” said Mr Jack, a former whip.
‘Fair and balanced’
Mr Johnson said the “great new deal” would see the UK “take back control of our laws, borders, money and trade without disruption”.
It removes the much-disputed “backstop” proposals for the Irish border post-Brexit, and would instead see Northern Ireland remain in the UK’s customs territory – while adhering to a limited set of EU rules on goods. Representatives in Northern Ireland would be able to decide whether to continue this arrangement every four years.
European Commission President Jean-Claude Juncker said it was a “fair and balanced agreement” – and suggested that it was the final deal on offer, saying there would be “no other prolongation”.
However, opposition parties in the UK have been critical, with Labour leader Jeremy Corbyn saying the deal sounded “even worse” than what was negotiated by the previous prime minister, Theresa May.
The Scottish Conservatives said the “onus” was on Ms Sturgeon and her MPs to back the deal, saying it would be “unforgivable” if opposition parties “put their narrow party interests, grievances and ambitions over the best interests of the country”.
Ms Sturgeon also repeated her call for a second independence referendum to take place in 2020, saying it was “clearer than ever that the best future for Scotland is one as an equal, independent European nation”.
She told her party conference on Tuesday that she would submit an official request to the UK government for an agreement to hold such a referendum by the end of this year.
However, the UK government has repeatedly said it will not do such a deal, saying the 2014 ballot was a “once in a generation decision”.