Assembly Members are expected to give the thumbs down to the law implementing Boris Johnson's Brexit deal in a vote in the Senedd today.
But the UK Government has already said it will ignore the vote - as it has the right to do - and the Welsh Government's position has been described as 'embarrassing' by the Assembly's Conservative leader.
AMs will be asked to give or withhold their consent to changes in the law which would affect Wales and which will be introduced by the Withdrawal Agreement Bill currently going through parliament.
They're expected to refuse consent because Labour and Plaid Cymru are planning to vote against and they have the most seats in the Senedd.
The UK Government's Brexit Secretary has written to his counterpart in Wales urging him to change his mind and offering a concession on a future say in the make-up of a crucial post-brexit government body.
It's thought that won't be enough to head off a refusal of consent and Wales will become the third devolved nation to say 'no' to the legislation as members of Northern Ireland's assembly did yesterday and MSPs did in a recent symbolic vote.
Normally the UK Government respects such refusals under a principle known as the Sewell Convention. However the Supreme Court ruled last year that Sewell is just a convention and not legally-binding so Boris Johnson isn't obliged to take them into account.
Even so the fact that ministers in London are prepared to override the views of all three devolved legislatures is unprecedented and has the potential to lead to a constitutional crisis, something that the Welsh Government, unlike its Scottish counterpart, wants to avoid.
In his letter, Steve Barclay goes some way towards trying to defuse the situation:
I understand that there have been intense private talks between the Welsh and UK Governments at ministerial and official level to try to find a way through.
They haven't been enough to avoid today's likely rejection but the words in Steven Barclay's letter suggests that despite the public disagreement the Welsh Government is receiving enough assurances to avoid this becoming an outright confrontation.
While Labour and Plaid Cymru are set to vote against consent in the Assembly today, the Conservatives and the Brexit Party are likely to vote for it.
The Welsh Tory leader Paul Davies denied that opposition from all three devolved administrations would be embarrassing for his party, saying instead that it's Labour and the SNP who should be embarrassed.
But one of the parties intending to vote to refuse consent says its worried the Withdrawal Agreement Bill will take powers away from Wales and cause further uncertainty for businesses.