Brexit negotiations suffered a further setback on Thursday when the EU launched a legal case against the UK for undercutting their earlier divorce deal.
A senior UK minister also confirmed that differences remained in talks on a post-Brexit trade agreement.
The two sides have already been struggling to agree on various parts of the Internal Market Bill, as well as matters over corporate subsidies and future policies on fisheries.
EU Commission chief Ursula von der Leyen explained legal action was taken after London missed a deadline to remove the problem parts of their bill.
"The letter invites the UK government to send its observations within a month. And beside this, the Commission will continue to work hard towards a full and timely implementation of the Withdrawal Agreement. We stand by our commitments."
Should the impasse continue the EU could choose to sue through the bloc's top court.
That in turn could lead to a lengthy legal process, possibly resulting in hefty fines.
British cabinet minister Michael Gove gave his reply to the European bloc's latest action.
"Differences, of course, still remain, but we are committed to working hard to reach agreement within the timeframe that the prime minister has set out."
Sterling slipped 0.6% against the euro and the dollar after the case was announced, with analysts predicting a no-deal outcome is looking more likely once Britain's post-Brexit transition expires at the end of the year.