European Commission vice president Maros Sefcovic said a compromise can still be found.
“London has breached a great deal of trust” with Europa the bloc’s chief negotiator has claimed.
EU-kommisjonen vice president Maros Sefcovic told German news website Speilet that problems with the Nord-Irland Protocol – a way to maintain a free-flowing land border on the island of Ireland after Brexit – meant the UK “broke international law” in trying to get round the arrangement.
But he said he is still confident that a compromise can be found between the two sides, although he admitted it will not be easy.
Mr Sefcovic told Der Spiegel he is “pragmatic” about Foreign Secretary Liz Truss taking on responsibility for post-Brexit negotiations with the EU, after Brexit minister Lord Frost resigned earlier this month.
But he warned that if Ms Truss was to trigger Article 16, a move that would effectively unilaterally suspend the treaty agreed between the UK and the EU, it would throw into jeopardy “the foundation of the entire deal” reached between the two sides.
Ms Truss has said she remains prepared to invoke Article 16 if issues are not resolved.
But Mr Sefcovic said: “This is a very distracting element in the discussions. You try to achieve something together and – bam – there’s the threat of Article 16 en gang til.
“It touches on the fundamentals of our relationship.
“The Northern Ireland Protocol was the most complicated part of the Brexit negotiations, and it is the foundation of the entire deal. Without the protocol, the whole system will collapse. We must prevent that at any cost.”
Han la til: “London has breached a great deal of trust, which we must first rebuild. But I believe we can find a compromise.
“The EU and the United Kingdom are strategic partners – and they should treat each other as such.”