The SNP is to call for a three-month extension to Brexit to allow time to hold a general election.
Ian Blackford, the party’s Westminster leader, has tabled an amendment to Saturday’s motion in the Commons, rejecting the new Brexit deal.
He also calls for an extension until at least 31 January 2020, allowing for an early election.
Boris Johnson has said he is “very confident” MPs will back the Brexit deal he has struck with the EU.
They are due to debate the withdrawal deal at a special sitting of Parliament on Saturday.
Earlier, first minister Nicola Sturgeon warned that the prime minister’s deal would lead to a “much harder Brexit” than earlier plans.
Mr Blackford said Mr Johnson’s Brexit deal would be “devastating for Scotland”.
He told the BBC’s Good Morning Scotland programme: “We will have all 35 SNP MPs in Westminster and will certainly be voting against this deal.
“This is a disaster for Scotland. It weakens our economy, takes us out of the European Union, takes us out of the single market and the customs union.”
Mr Blackford also called on opposition parties to “quit dithering, back our amendment, and finally act to bring this appalling Tory government down and stop Brexit”.
He said: “I would simply say to those on the Labour benches, don’t be the midwife of a Tory Brexit.
“I hope it is the case that we defeat this tomorrow. It won’t be the end of the road if it goes through, because the government has to bring a bill forward.
“But it would be very significant…and pretty devastating if the government were to get this through on a small number of Labour votes.”
Earlier, the first minister said it was “clear that Scotland is being treated unfairly”, and confirmed that SNP MPs “will not vote for Brexit in any form”.
After an agreement between the UK and EU was announced on Thursday morning, Ms Sturgeon said a “much harder Brexit beckons if this deal passes”.
It is unclear if the new deal will pass a vote of MPs, with the DUP saying they still cannot support it.
Mr Johnson said the “great new deal” would see the UK “take back control of our laws, borders, money and trade without disruption”.
‘Fair and balanced’
The deal was announced by Mr Johnson and European leaders via Twitter on Thursday morning, ahead of a summit in Brussels.
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”.
Mr Johnson said the “great” new deal “allows us to get Brexit done and leave the EU in two weeks’ time, so we can then focus on the people’s priorities and bring the country back together again”.
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”.
And Ms Sturgeon said Mr Johnson’s plan would lead to a “much harder Brexit”, highlighting that people in Scotland voted for Remain by 62% to 38% in the 2016 poll.
The SNP leader had always been clear that her 35 MPs would reject any deal brought back by Mr Johnson which takes the UK out of the EU’s single market and customs union.
Reiterating this on Thursday, she said: “We support efforts to ensure peace and stability on the island of Ireland, in line with the Good Friday Agreement, which must be respected.
“At the same time, it cannot be right that Scotland alone is facing an outcome it did not vote for – that is democratically unacceptable and makes a mockery of claims that the UK is in any way a partnership of equals.
“The Brexit envisaged by Boris Johnson is one which sees a much looser relationship with the EU when it comes to issues like food standards, environmental protections and workers’ rights. That is not the future that I or my government envisage for Scotland.”
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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”.
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