Theresa May is facing a fresh Brexit wrangle as the Scottish parliament is expected to formally refuse its consent to key legislation after months of rows over its impact on devolved powers.
MSPs will set themselves at odds with Westminster by backing a Scottish government motion that makes clear that Holyrood "does not consent to the EU Withdrawal Bill".
The move could trigger a constitutional crisis, as it would be the first time the UK government has pushed through laws against the will of Scotland, if Ms May decides to press ahead with the legislation.
It comes as the prime minister struggled to break the deadlock over customs arrangements after Brexit, which have divided her cabinet.
Scottish Labour, the Greens and the Liberal Democrats have said they will back the SNP in the Holyrood vote later today, which would give Nicola Sturgeon's party the numbers to defeat the Scottish Conservatives on the motion.
Scottish government Brexit minister Mike Russell said the bill "rides roughshod over devolution", as it could see some powers returned to Whitehall, rather than Holyrood, after the UK leaves the EU.
"Devolution is the settled constitutional will not just of the Scottish people, of the whole of the UK," he told the BBC's Today programme.
"Westminster is trying to subvert that and to change that by the back door."
Such a move would not prevent the UK government from introducing the legislation - but it would be the first time Westminster has pushed through laws against the wishes of the Scottish Parliament.
Tory MSP Adam Tomkins said: "The Scottish Conservatives will vote to give our consent to the amended Withdrawal Bill today.
"It's profoundly regrettable that we don't have a deal in Scotland to allow us to move on.
"The blame for that lies entirely with the SNP. Nicola Sturgeon has refused to compromise. It's not in Scotland's interests that the SNP prefers picking fights to making a deal."
However David Mundell, the Scottish secretary, rejected claims of a "power grab" as he said no changes were being made to existing arrangements.
He said: "Obviously, it's up to the Scottish parliament to take whatever decision they want to take.
"I still hope that they will take a positive decision, that they will take a step back and look at the proposal that we have put forward in relation to the specific arrangement which is just to keep 24 areas which affect the whole of the UK exactly as they are at the moment, so that the day after we leave the EU nothing will have changed.
"Then there will be an opportunity to have a discussion across the whole of the UK as to what new arrangements should replace those which are already in place. I just don't see how that can be characterised as a power grab."
It comes ahead of a meeting of Ms May's Brexit war cabinet, where senior figures are trying to find a way to end the standoff over the future customs relationship with the EU.
Ministers have been split into two groups to explore the two options - a "customs partnership", which is favoured by Ms May, and "maximum facilitation" plans backed by Brexiteers.
The EU is pressing for "substantive progress" from Britain on the future of the Irish border in time for a crunch European Council summit just six weeks away.
