Northern Ireland's place in the EU's single market is in jeopardy, the EU's chief Brexit negotiator has told Euronews.
Maros Sefcovic's warning comes amid a long-running dispute between London and Brussels over the Northern Ireland Protocol.
He said the UK had failed to respond to solutions put forward by Brussels to resolve issues with the protocol, which was drawn up to govern trade with Northern Ireland, which as part of the Brexit agreement remains in the EU's Single Market.
To avoid a hard land border between Northern Ireland (part of the UK) and the Republic of Ireland (part of the EU) -- extremely sensitive because of sectarian tensions -- customs checks on certain goods, it was agreed, would instead be done as they arrived into the north from Britain.
However, the UK says these arrangements are not working; the EU proposed reforms in October but London has rejected the plan.
"There was no engagement, no action from the UK side," said Sefcovic, adding that the bloc is "seriously concerned that as it seems, the UK Government is embarking again on the unilateral path".
He warned that Brussels has not ruled out a suspension of the Brexit trade deal -- or the EU-UK Trade and Cooperation Agreement (TCA) -- if the UK proceeds with tearing up parts of the protocol.
Asked if the TCA was under threat, Sefcovic told Euronews "once we see the concrete proposal from the UK government, of course, we will look for all alternatives; all options, and we'll discuss this with our member states and the European Parliament".
Sefcovic also warned Northern Ireland's place in the EU's Single Market is in jeopardy if the UK proceeds with unilateral action to breach the Northern Ireland Protocol.
"This puts a huge question mark over the access of Northern Ireland to the single market. It puts a huge question mark over all the arrangements we've been working so hard to make sure that we promote peace, stability and predictability on the island of Ireland without a hard border."
Protocol 'takes us backwards'
A phone call between Sefcovic and UK Foreign Secretary Liz Truss earlier in the day failed to de-escalate the situation.
The UK says it will put forward legislation to scrap parts of the Northern Ireland Protocol unless the EU gives in to its demands to end most regulatory checks on goods moving from Britain to Northern Ireland, arguing the current arrangements it negotiated and agreed to before the UK finalised its divorce with the bloc create a border in the Irish Sea.
Truss has said EU proposals don't "properly address the real issues affecting Northern Ireland and, in some cases, would take us backwards".
Following the phone call today, Truss insisted that if the EU doesn't "show the requisite flexibility to help solve the issues" the UK would "have no choice but to act".
Sefcovic rejected these claims saying "we are proposing 80% cuts if it comes to phytosanitary sanitary controls, more than 50% cut in the customs procedure".
"Again, we just got 'no' from London."
"They wanted to leave the Single Market, the Customs Union, to leave the European Union. And this is just the consequences of the Brexit the UK government has chosen to follow", he added.