The European Commission says it will start legal action against Boris Johnson’s bid to potentially override parts of the Brexit deal.
Brussels claims the prime minister has breached the “good faith” promise both sides signed up to in the withdrawal agreement struck and passed by parliament last year.
It is a major escalation in tensions as the deadline looms for negotiators to hammer out a trade deal.
The UK left the EU on 31 January but is in a transition period meaning it is following many of the same rules until the end of 2020.
Talks are now ongoing over the future relationship but if no breakthrough is reached by mid-October, then the prospect of a no-deal divorce will rear its head again.
It will let ministers hand themselves the power to determine rules on state aid and goods travelling between Northern Ireland and Great Britain.
Despite a small Tory rebellion, the bill passed the Commons with ease earlier this week and will go on to face debates and votes in the House of Lords.