The European Union on Friday (September 10) rejected a British demand to renegotiate their deal governing the trading position of Northern Ireland.
European Commission Vice-President Maros Sefcovic said in a speech that the Northern Ireland protocol needed to be properly implemented:
"A renegotiation of the protocol - as the U.K. government is suggesting - would mean instability, uncertainty and unpredictability in Northern Ireland".
Under the protocol, Britain agreed to leave some EU rules in place in Northern Ireland and accept checks on goods arriving from elsewhere in the UK.
London has since said the arrangement is not working and wants it changed.
Sefcovic, on a two-day visit to the British province, said the EU was seeking solutions for all, including those opposed to the protocol.
"I know it is possible for us to work together, if rhetoric on both sides is dialled down".
He added that any solutions could only minimise the effects of Brexit, not entirely remove them, given London's choice to leave the EU single market and customs union.
Northern Ireland's biggest pro-British party, the Democratic Unionists, called the comments "belligerent" and "foolish".