Scottish and Welsh politicians are joining forces in a bid to force the prime minister to change her position on Brexit.
An identical motion will be debated simultaneously by the Scottish Parliament and the Welsh Assembly – with co-ordinated votes.
It will underline opposition to Theresa May’s deal, demand a delay to Brexit and call for “no deal” to be ruled out.
The UK government said the deal was a good one for Scotland and Wales.
It said the focus should be on gaining further assurances from the EU to allow the prime minister’s deal to go forward.
Mrs May has promised MPs a vote to delay Brexit if they cannot agree on her deal.
Scottish Brexit Secretary Mike Russell said it was vital to prevent Westminster from pursuing “a disastrous course of action”.
And he said the delay could enable a further Brexit referendum to be held.
He 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.”
He added: “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.”
The debates in the devolved parliaments come UK ministers resumed their efforts to secure legally-binding changes to the prime minister’s Brexit deal that might get MPs’ backing in a week’s time.
UK Brexit Secretary Stephen Barclay and Attorney General Geoffrey Cox will meet EU officials in Brussels in search of guarantees over the backstop plan to avoid border checks in Ireland.
MPs will vote on the deal by 12 March, with the UK currently due to leave the EU on 29 March.
Leading Brexiteers are seeking assurances that the backstop – a controversial plan which will see the UK aligned with EU customs rules until the two sides’ future relationship is agreed or alternative arrangements worked out – will not endure indefinitely.
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.
“Refusing to support the prime minister’s deal simply makes a damaging no deal more likely.”