The SNP will not back the draft Brexit agreement between the UK and EU as it currently stands, the party’s leader at Westminster has confirmed.
Ian Blackford said it appeared the party’s “red lines” on Scotland remaining in the single market and customs union had not been met.
And he said any special arrangements for Northern Ireland must also apply to Scotland.
The prime minister is to host a cabinet meeting in Downing Street later.
But the Brexit deal proposals, which have not yet been published, are already facing strong opposition from senior Brexiteers and some Remain supporters.
The BBC’s Norman Smith said the prime minister would seek to head off the threat of any resignations by telling her ministers that while not perfect, the agreement was as good as it can get.
However the Democratic Unionist Party, which props up Theresa May’s minority Conservative government, has warned that the deal could lead to the break-up of the United Kingdom if it is accepted by MPs.
The major sticking point in the negotiations between the UK and EU has been over the so-called Northern Ireland “backstop”, which aims to guarantee that physical checks will not be reintroduced at the border with the Irish Republic if the EU and UK fail to agree a deal on future trading relations.
The draft agreement is believed to suggest keeping the UK as a whole aligned with the EU customs union for a limited time.
Some Brexiteers fear this will keep the UK locked into EU trade rules for years.
And Mr Blackford told the BBC’s Good Morning Scotland programme that it appeared there would also be some protections for the single market in Northern Ireland that would not apply to the rest of the UK.
Mr Blackford said: “I applaud the fact that we recognise the individual circumstance of Northern Ireland.
“But the key point is that if there is to be a differentiation for Northern Ireland there is no reason why the circumstances in Scotland can also not be respected.
“We need to stay in the single market and the customs union and the government in London must listen to us on that.”
Sources told the BBC last month that Scottish Conservative leader Ruth Davidson, who is currently on maternity leave after the birth of her son, and Scottish Secretary David Mundell could quit their roles if a Brexit deal introduced different arrangements for Northern Ireland.
Mr Blackford said the two Tories had “got questions to ask”, but added: “I’ll leave that to them.”
Mr Blackford insisted that the choice facing MPs was not between the current draft deal or no deal at all, and backed the prospect of a so-called “People’s Vote” referendum on the final Brexit terms.
But he said: “We need to make sure that if Scotland again votes very heavily to Remain, as I expect it would, that our position is protected.
“We cannot, we must not, be dragged out of the European Union and the single market and the customs union in particular against our will. Scotland has got to be able to determine its own future if that is the case.”
‘Phoney war has ended’
In a separate BBC interview, he added: “All of us as parliamentarians have now got to reflect on where we are. The phoney war is ended. This is now about the future of all the nations of the United Kingdom.
“It’s about the government’s desire to take us fundamentally out of the single market and the customs union. We know that that’s going to cost jobs.
“I would say to every MP think about what you’re doing. Think about your constituents’ interests, think about jobs, think about prosperity
On Tuesday, Mr Blackford joined Labour leader Jeremy Corbyn, Lib Dem Sir Vince Cable and Plaid Cymru’s Liz Saville Roberts in writing to the prime minister to seek assurances that MPs would be given a “meaningful vote” on the Brexit agreement.
This would mean that amendments to be proposals can be tabled and voted on.
Their joint letter stated: “Recent interventions from government ministers have suggested that you and your government may seek to limit or constrain the process on the final vote, in an attempt to muzzle parliament.
“We want to be clear that this would be wholly unacceptable.”
What happens next?
The prime minister is continuing one-to-one briefings with her ministers on the plans ahead of a full cabinet meeting at 14:00 that is expected to last three hours.
If the cabinet signs it off, the EU Commission is expected to publish the details of the 500-page draft withdrawal agreement later, as well as the much shorter declaration on future economic and security relations.
Ambassadors from the remaining 27 EU member states will discuss the possibility of organising an emergency summit later this month, earmarked for 25 November, to sign off on it.
If this happens, the government will then face a battle to win Parliament’s backing, with some Tories vowing to vote against it and Northern Ireland’s Democratic Unionist Party – which Mrs May relies on to win key Commons votes – also expressing concern.
Labour have said they will study the agreement before deciding whether to back it, but leader Jeremy Corbyn has said it is “unlikely to be a good deal for the country”.