European Commission vice-president Maros Sefcovic has accused the UK of failing to implement a range of obligations under the Northern Ireland Protocol, and says that sweeping changes to the protocol will not be possible.
In a letter to his UK opposite number Michael Gove, Mr Sefcovic said any flexibility the EU was prepared to offer on how the protocol is implemented will depend on the UK complying with what was agreed by both sides in December.
He said "blanket derogations" from EU law in Northern Ireland covering certain meat and animal-derived products "cannot be agreed beyond what the Protocol foresees already".
In the letter, Mr Sefcovic said border control posts at Belfast and Larne ports were "not yet fully operational" and that controls being carried out there by Northern Ireland veterinary officials were "not in compliance with the Withdrawal Agreement".
He said there were "very few identity checks" and, other than those carried out on live animals, "a very limited number of physical checks" on consignments entering Northern Ireland from Britain.
Mr Sefcovic, who is the EU's chair on the Joint Committee, which oversees the implementation of the Protocol, wrote that "all non-compliant consignments are accepted [at Northern ports], even if destined for Ireland [ie the single market]".
Furthermore, packages were "not labelled and the packages not monitored at destination," as required by the 17 December agreement between the EU and UK.
"In addition, a number of consignments are entering Northern Ireland without being declared or without valid certificates," he wrote.
The letter is in response to a letter from Cabinet Office minister Michael Gove last week which demanded sweeping changes to the protocol, including extending grace periods for controls on products of animal origin until 1 January, 2023.
Mr Sefcovic also said that, contrary to the 17 December agreement between both sides, EU representatives were still being denied real time access to the UK's IT systems, such as the import clearance system, or CDS.
He writes: "We are thus not currently receiving the information as to how mutually agreed flexibilities including, eg, the trusted trader scheme, or simplified health certificates are being used in practice."
Mr Sefcovic goes on to accuse the UK of telling traders sending goods from Northern Ireland to Britain that they do not have to submit "equivalent information" to an exit summary declaration which, he said, was "contrary to the commitment taken by the UK" in a unilateral declaration on 17 December.
Both men are due to meet in what is expected to be a tense meeting tomorrow in London.
EU officials and member states have been hardening their position on what flexibilities they are prepared to grant the UK in implementing the protocol.
There is a growing view that the UK has attempted to exploit the controversy over the commission's move to trigger Article 16 of the Protocol on 29 January in order to force changes to the protocol.
Mr Sefcovic said he was "convinced" that the alleged non-compliance by the UK was down to "teething problems" which should be addressed through the Joint Committee.
He said the EU was working on a solution to a recent anomaly which would see steel imports into Northern Ireland from both Britain and the rest of the world hit by tariffs.
Mr Sefcovic said there could be no "blanket derogations" from EU rules when it came to export health certificates for products of animal origin, certain meat products and parcel deliveries beyond that already foreseen in the protocol.
He also said that if the UK wanted to agree a common travel area for pets moving between Britain and the island of Ireland, and for the lifting of the ban on seed potatoes "and other plants and plant products", then "any flexibility would entail the UK committing to align with the relevant EU rules".
A UK government spokesperson said: "It is disappointing that the Commission has failed to acknowledge the shock and anger felt right across the community in Northern Ireland from its decision to trigger Article 16, and the need to take urgent steps to restore confidence as a result.
"The Chancellor of the Duchy of Lancaster will underline the need for such action and political leadership in this regard when meeting with Vice-President Sefcovic in London tomorrow evening."