EU chief Brexit negotiator Michel Barnier | John Thys/AFP via Getty Images Michel Barnier: UK Brexit plan ‘undermines’ single market The Brexit negotiator says the UK ‘cannot ask the EU to lose control of its borders and laws.’

LONDON — Theresa May’s Brexit plan would “undermine” the European single market and the legal order of the EU, Michel Barnier said in his firmest rejection of the U.K.’s new position.

In an article published in several European newspapers, the EU’s chief Brexit negotiator said that he remains “confident negotiations can reach a good outcome,” but indicated that key pillars of May’s proposal are unacceptable.

The compromise plan — thrashed out by the U.K. Cabinet at May's Chequers country residence amd formally set out in a white paper — proposes the U.K. follow EU rules and regulations governing the goods trade, while envisioning a more distant relationship on services trade, and an end to free movement of people. It also asserts that the U.K. continue to levy EU tariffs at U.K. borders on imports destined for the bloc.

However, Barnier said that these proposals — core facets of the U.K.’s vision for a future economic partnership — create problems for the EU.

“Some UK proposals would undermine our Single Market which is one of the EU's biggest achievements,” he writes. “The UK wants to keep free movement of goods between us, but not of people and services. And it proposes to apply EU customs rules without being part of the EU's legal order. Thus, the UK wants to take back sovereignty and control of its own laws, which we respect, but it cannot ask the EU to lose control of its borders and laws.”

However, Barnier said that on the key outstanding issue of the withdrawal phase of the negotiation — guaranteeing no hard border between Ireland and Northern Ireland — the EU was “ready to improve the text” of its proposal for a “backstop” arrangement.

Peter Ptassek, the German government’s Brexit coordinator, singled out “the crucial question of Northern Ireland” in a tweet responding to Barnier’s article, adding that he agreed a deal could be reached if this issue could be solved.

However, given the EU’s stated position on Chequers, reaching agreement on the future relationship would require further concessions from the U.K.

Barnier also signaled the EU’s willingness to maintain “very close cooperation” with the U.K. on security and foreign policy matters, noting that the EU had an “interest” in keeping the U.K. as a “close partner” with “common values” and a “a number of common interests.”