An attempt is being made by Tory MP Steve Baker to render illegal the EU’s Northern Ireland Brexit backstop, which would put Northern Ireland in the customs union and single market but not Great Britain.
The former Brexit minister has put down amendments to the Northern Ireland bill to that effect.
This gives the government and MPs a massive dilemma.
On the one hand, Theresa May and most MPs are implacably opposed to seeing Northern Ireland alone in the trading area with the EU, separate and different from that relating to Great Britain.
That's because it would put a new border or barrier between Northern Ireland and Great Britain.
But the EU reaffirmed on Thursday that this backstop, with no end date, must be in the Brexit Withdrawal Agreement.
So if Parliament were to vote to render the backstop illegal, it would be voting for a no-deal Brexit.
Baker is forcing the prime minister to choose between the integrity of the UK and a negotiated Brexit.
This is her worst nightmare.
What the leading Brexiter, Baker - who organises the European Research Group of Tory MPs - is doing more precisely is making it impossible for the backstop to be introduced without the approval of the Northern Ireland Assembly.
His reasonable presumption is that this would never happen because a) it is in abeyance with no timetable to be restored and b) even if it was restored the DUP would never vote for it.
But in making the backstop only applicable if the devolved government gave assent, he makes it harder for May to reject his amendments.