In unusually blunt language, UK negotiator David Frost on Tuesday accused the EU of offering a "low quality trade agreement" that he said was not worthy of a close and sovereign partner like Britain.
"I would not like the tone that you have taken to impact the mutual trust and constructive attitude that is essential between us," Michel Barnier said in a letter to Frost.
Barnier said he "did not think an exchange of letters regarding the substance of negotiations is necessarily the best way forward".
The ill-tempered exchange came just days after a third round of video trade talks ended with no progress, with each side accusing the other of digging in on its positions.
In his three-page letter, Barnier reviewed the EU's long-known stance in the talks and rejected the argument that Brussels was denying Britain the types of benefits it gave other countries in trade accords.
The former French minister insisted that his teams were faithfully following the terms of a political declaration signed with London that laid out a roadmap for future ties.
"This is the only precedent that the EU is following," Barnier wrote, calling any new agreement "unique" and "tailored to the partner and era it was concluded".
Frost argues an eventual free trade agreement could closely follow the model of deals like the EU has with Canada, and questioned why Brussels is seeking more oversight of trade with Britain.
But Barnier firmly defended Europe's pursuit of the "level playing field" -- a guarantee that Britain will uphold EU standards on the environment, health, state aid and other norms.
This has become one of the most contentious aspects of the talks -- along with Europe's desire to retain its current fishing rights for EU vessels in British waters.
Britain left the EU in January, nearly four years after a landmark referendum in June 2016 that saw voters choose to end nearly 50 years of integration with Europe.
It remains bound by EU rules until December 31 as it tries to secure terms for a new relationship with its biggest trading partner.
A fourth round of negotiations is set for June 1, just weeks ahead of a deadline for an extension to the talks -- an extension that Prime Minister Boris Johnson has emphatically ruled out.