As talks take place on how to overhaul the divorce terms, the bloc's ambassador to the UK laid down a marker that it is not prepared to give much more ground.
The combative stance came after the European Commission unveiled proposals to slash 80 per cent of regulatory checks and dramatically cut customs processes on the movement of goods between mainland Britain and Northern Ireland.
Although the government welcomed the shifts outlined by Maros Sefcovic - which should ease the flow of food, farming produce and medicines - there is still a major flashpoint over the role of the European Court of Justice in policing the arrangements.
Brexit minister Lord Frost has insisted the ECJ should be removed entirely from the process and and independent arbiter should resolve disputes.
But there are hints at a compromise on that issue as well, with reports that the EU is ready to water down the court's role so it only gets involved as a final resort to settle spats over narrow legal interpretation.
EU officials are in London for talks today, while Lord Frost is expected in Brussels tomorrow to meet Mr Sefcovic.
Speaking on BBC2's Newsnight, EU ambassador Joao Vale de Almeida said Brussels has gone the 'extra mile' and cannot go any further following Wednesday's proposals
The Northern Ireland protocol has sparked a rise in sectarian tensions in the province
EU officials are in London for talks today, while Lord Frost (right) is expected in Brussels tomorrow to meet Maros Sefcovic (left)
EUROPEAN COURT OF JUSTICE
The proposals by the European Commission offered no progress on this issue. Brexit minister Lord Frost has made clear the removal of the oversight function of the European Court of Justice (ECJ) in policing the protocol is a red line for the Government.
The UK wants to remove a provision that gives the ECJ the final say in any future trade dispute and to replace it with an independent arbitration process.
Failure to move forward on this key issue threatens to blow up the talks.
However, there are hints at a compromise that could reduce the powers of the ECJ, ensuring they can only rule on 'narrow' legal matters once an independent process has been exhausted.
The EU plans would see 80 per cent of checks removed on goods moving from Great Britain to be sold on supermarket shelves in Northern Ireland.
Lorries transporting different food products would only need one certificate stating all the goods rather than a separate one for each type.
An EU ban on the import of chilled meats, which would have prevented sausages and burgers entering Northern Ireland from Great Britain, will end thanks to a special exemption.
Brussels is proposing a new category of ‘national identity’ products to include British produce.
Paperwork will be halved for goods arriving into Northern Ireland from Britain under the EU’s offer.
For example, a car dealer in Belfast buying parts from London will only need to provide basics such as the total value of the shipment rather than detailed information.
The EU will allow medicines licensed for sale in the UK to be prescribed in Northern Ireland without having to undergo further checks by European regulators.
Brussels promised to change legislation to ensure no disruption to medical