Barnier has sent a desperate warning shot to Theresa May hoping to maintain his support from the remaining EU27 as the Prime Minister continues her diplomatic assault on Europe.
He has been busy trying reaffirm his position as the EU’s chief negotiator by countering Britain’s attempt to secure a change in the bloc’s negotiating position.
Writing in over 20 European newspapers, the Frenchman repeated a number of rejections to the customs proposals to collect trade tariffs on behalf of Brussels, which was featured in Mrs May’s controversial White Paper.
However, he presented a more conciliatory tone when discussing the EU’s willingness to improve on its offer for the Irish backstop.
Mr Barnier’s intervention comes as Mrs May and her ministers launch a continental charm offensive in the hope of securing a number of concessions from EU27 leaders.
Brussels’ negotiator said: “The UK knows well the benefits of the Single Market. It has contributed to shaping our rules over the last 45 years. And yet, some UK proposals would undermine our Single Market which is one of the EU’s biggest achievements.
“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.”
The EU’s demand for a backstop – a mechanism to prevent the hardening of the Irish border – has plagued negotiations, with both parties insisting each others’ offer in unacceptable.
Mr Barnier has confirmed he is ready to offer Britain an improvement on the EU’s offer, which currently would see Northern Ireland remain in the bloc’s customs union until a permanent solution is found.