The European Union says Britain is trying to weaken the bloc's position at trade talks by talking to individual member states and has urged EU governments not to hold a "parallel negotiation" with London.
An EU document seen by Reuters said the bloc's negotiating stance could be undermined if member states talk separately to Britain while a new trade agreement with London is being negotiated following its departure from the Union.
Formal negotiations began in Brussels this week, just over a month after Britain left the EU. The aim is to reach agreement on a new trade relationship by the end of this year.
"We have information that the UK colleagues are approaching us at different levels and trying to engage in issues that are linked to our negotiations," a senior diplomat from Croatia, which chairs EU meetings in the first half of 2020, told a meeting of Brussels envoys from member states.
"(We) Would like to ask (member states) to restrain from engaging in contacts of a nature that may be perceived as a parallel negotiation, in substance, and perhaps undermine the position of the EU and negotiator," read the minutes from Wednesday's meeting.
The EU's executive European Commission is in charge of the talks on behalf of the 27 member states, and Frenchman Michel Barnier is the chief negotiator.
Asked to comment, a British government spokesman said: "The UK is clear that it is negotiating its future relationship with the EU with the European Commission, and it is for the European Commission to find common ground among the EU's member states."
The EU has long had concerns that Britain will try to divide member states to try to win the upper hand in the negotiations.
Similar tensions were on display in more than three years of tortuous divorce talks which led to Britain leaving the bloc in January but the EU maintained united position.
The EU is now negotiating simultaneously a deal on trade in goods with Britain, a new security pact and agreements on a wide range of issues from transport to fisheries.
It is also separately assessing its future financial ties with London, the largest financial center in Europe.
EU governments were warned on Wednesday that Barnier "should be the sole interlocutor for the EU," according to the minutes.