Rishi Sunak returns to London this morning after spending two days in Scotland.

His visit included a private dinner with Nicola Sturgeon on Thursday evening, during which the pair discussed, among other things, the many issues that divide them.

Nevertheless, the prime minister said he wanted the pair to “strengthen our working relationship and continue discussing our shared challenges and our joint efforts to deliver for people in Scotland”.

But that laudable intention will be blown to pieces next Wednesday if, as HuffPost UK understands, Sunak’s government takes the unprecedented step of blocking a bill passed at Holyrood from becoming law.

The legislation in question is the Gender Recognition Reform Act, which was passed by the Scottish Parliament before Christmas after a fierce debate which highlighted splits on the issue in the main parties.

In simple terms, the new law would make it easier for trans people to change gender by reducing the time the process takes and lowering the age at which it can happen.

The Westminster government has no intention of bringing in a similar bill, giving rise to potential legal problems over how the Scotland-only law would affect the application of the Equalities Act across the UK.

Ministers in London can block Holyrood legislation in such circumstances by triggering section 35 of the Scotland Act — which brought the Scottish Parliament into being in 1999.

That would prevent the presiding officer of the Scottish Parliament from sending the bill to the King for royal assent.

Whitehall sources say Alister Jack, the secretary of state for Scotland, will announce on Wednesday that the UK government will block the bill.

Coming at a time when the SNP are already accusing Westminster of “denying democracy” by not allowing another referendum on independence, this would be the political equivalent of throwing a match into a box of fireworks.

Asked about the issue while in Scotland, Sunak was giving little away.

“Obviously this is a very sensitive area and I know there were very robust debates and exchanges on it as the bill was passing in in Scotland,” he said.

“What I’m concerned about is the impact of the bill across the United Kingdom.

“There may be impacts across the UK that we need to be aware of and understand the impact of them, and that’s what we’re doing, and once the government has received final advice it will set out next steps.”

Stephen Flynn, the SNP leader at Westminster, gave a foretaste of what his party’s reaction will be if the UK government does indeed veto the Gender Reform Recognition Act.

He said: “This goes to the heart of Scotland’s democracy because Scotland’s democratically elected parliament has approved legislation in relation to gender recognition and the UK government is potentially going to try and undermine that. That’s simply not on.”

It’s not a straightforward issue for the SNP, however.

The debate in the Scottish Parliament saw the biggest rebellion against the party leadership since it came to power in 2007, with some SNP MSPs voting against what they see as an attack on women’s sex-based rights.

The SNP’s Westminster group is also split, with Edinburgh South West MP Joanna Cherry one of the most vocal opponents of the Scottish government’s position.

Will those SNP rebels back the UK government blocking a law they hate, or will they row in behind Sturgeon over what their party claims is an attack on devolution itself?

One constitutional expert told HuffPost UK that the row will ultimately end up in the Supreme Court, which ruled in November that the Scottish Parliament does not have the power to organise its own independence referendum.

“Using such a radical mechanism as a veto power does rather play into the SNP narrative that Westminster is denying democracy, which seems to me to be politically risky,” they said.

“As we approach the 25th anniversary of devolution, it is a big moment.”

For all his warm words about co-operation with the Scottish government, it seems all-but certain that Sunak is just days away from a constitutional crisis which could end up defining the very future of the United Kingdom.