The UK has complained that the EU is failing to attend to the rights of UK citizens living in member states, claiming the EU is at "serious risk" of not fulfilling its obligations under the Withdrawal Agreement.
It follows the threat of legal action by the EU over complaints that the UK is taking an unduly strict approach to the free movement of EU citizens already living in the UK.
The British complaints relate to an alleged failure by national capitals to properly inform British citizens about their rights post-Brexit, and to give them ample time to apply for settled status.
London says EU member states are insisting on complex processes when it comes to applying for settled status, and of operating an unclear timetable.
The UK also claims that due to what it calls the "misapplication" of the Withdrawal Agreement by the EU, a number of British nationals have been denied their rights on EU health cards, the exchange of driving licences and the purchase of property.
The UK says British nationals are required in some member states to attend physical interviews which may be impossible due to the coronavirus emergency.
In a letter from the British cabinet secretary Michael Gove to his opposite number in the European Commission, Maros Sefcovic, Mr Gove says member states have failed to inform British citizens in their countries as to what financial support they are offering elderly, vulnerable or hard-to-reach UK nationals.
Mr Gove insists that while the alleged failings are attributable to member states, Article 37 of the divorce treaty obliges the EU as a whole to take account of the needs of UK citizens in EU member states.
In the letter, Mr Gove says the UK has provided EU citizens in Britain with simple, cost-effective, and streamlined processes for applying for settled status.
Some 3.4 million applications have been registered, with 58% of those applications qualifying for settled status, Mr Gove said.
In the letter, Mr Gove states: "When viewed cumulatively these themes amount to a serious risk that the EU will not fulfil its obligations under the Withdrawal Agreement by the time the transition period ends on 31 December."
There was no immediate response from the European Commission.
It is understood the EU’s chief negotiator Michel Barnier will address the issue during a news conference, which will conclude the latest round of negotiations on the future relationship.
The commission published a 57-page guidance on how to implement the citizens rights element of the Withdrawal Agreement for member states earlier this week.
Tonight's letter from Mr Gove comes amid growing tension over the implementation of the Withdrawal Agreement.
Today, the European Commission issued infringement proceedings against the UK over the free movement of people issue.
It has complained that the UK is too strictly imposing re-entry bans on European citizens who are deported, setting unfair restrictions on the right of EU citizens' family members to live in the UK, and making it too difficult to claim jobseeker's allowance.
The UK has said it is studying the infringement letter.