The European Union Thursday took legal action against Britain regarding its proposed Internal Market legislation that would breach sections of the exit agreement the nation signed with the bloc last year.
At a news conference in Brussels, EU Commission President Ursula Van Der Leyen said the EU had given Britain until the end of September to remove the problematic language, but that the deadline had not been met.
The Internal Market Bill currently moving through the British Parliament would ensure free, open trade among Britain's four "home" nations - England, Scotland, Wales and Northern Ireland - in the event no trade deal is established with the EU before Brexit takes effect at the end of the year.
The Republic of Ireland voted to stay in the EU.
Von der Leyen said the draft bill breaches sections of the Withdrawal Agreement and would contradict the Ireland/Northern Ireland protocol regarding free and open borders between the two countries.
The British government does not deny this, saying the bill is necessary to avoid a so-called "hard" border between the two Irelands that would impact the flow of trade in the event no trade deal is established.
An open border between Northern Ireland and the Republic of Ireland was a key component of the Good Friday peace agreement, signed in 1998, that ended three decades of violence in Northern Ireland.
London now has one month to reply to a formal letter of complaint from the commission, which will then assess whether the answer is satisfactory. The Reuters news agency says the EU can then request that Britain fall back in line. If that fails, the EU can sue at the Luxembourg-based European Court of Justice.
Meanwhile, British and EU negotiators are still trying to hammer out a trade deal in talks that have been overshadowed by the controversy over the Internal Market Bill.