The Scottish Parliament and the National Assembly for Wales will vote simultaneously on a motion declaring opposition to Theresa May's Brexit deal.
Both motions object to the deal to leave the European Union by the UK Government and declare that a no-deal Brexit would be "completely unacceptable".
If passed, they would call for the UK Government to "take immediate steps to prevent the UK leaving the EU without a deal" as well as an extension of Article 50.
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Scotland's Constitutional Relations Secretary Mike Russell said: "This is an unprecedented event: the first time in 20 years of devolution that the Scottish Parliament and National Assembly for Wales have debated the same motion simultaneously.
"We are taking this historic step to send a strong message to the UK Government that it must stop pursuing such a disastrous course of action.
"The Prime Minister's deal will cause major, lasting damage to jobs, living standards and public services such as the NHS, and should be voted down.
"The UK Government must also stop using the threat of a catastrophic no-deal outcome to blackmail the UK Parliament into accepting her deeply damaging plans.
"An extension that stops the clock on Brexit would allow time for agreement to be reached on a better way forward, which the Scottish Government believes should be a second EU referendum with Remain on the ballot paper."
A UK Government spokeswoman said: "An orderly Brexit is in the UK's best interests and the best way to achieve that is for MPs of all parties to support the Prime Minister's deal.
"The deal is a good one for Scotland, Wales and the whole of the UK - it delivers the result of the referendum, gives us a close future partnership with the EU, and guarantees citizens' rights.
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"Refusing to support the Prime Minister's deal simply makes a damaging no-deal more likely."
Scottish Conservative interim leader Jackson Carlaw MSP added: "At a time when most people want to see parties compromising in order to deliver an orderly Brexit, the SNP is still engaged in game-playing and stunts.
"This debate has little to do with moving Scotland and the UK forward and everything to do with Nicola Sturgeon using Brexit to push for independence. That is her only Brexit priority.
"Labour - in Wales and Scotland - should know better than to play along."
The vote is expected to take place at 6pm in Edinburgh and Cardiff.
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