Michel Barnier has said ???something has to give??? on the British side of the negotiations if the Brexit impasse is to be broken.
The EU???s chief negotiator insisted there was no question of Brussels giving in to Downing Street???s demands on the Irish backstop.
???We???re waiting for clarity and movement from the United Kingdom,??? Barnier told reporters after talks in Luxembourg with the country???s prime minister, Xavier Bettel.
At a dinner with the EU official in Brussels later on Monday, the Brexit secretary, Stephen Barclay, was expected to push Theresa May???s calls for a time-limit on the operation of the backstop, a unilateral exit mechanism or its replacement with an ???alternative arrangement???.
The prime minister argued last week in meetings with Jean-Claude Juncker, the president of the European commission, and his counterpart in the European council, Donald Tusk, that parliament???s support for the Brady amendment two weeks ago showed that legally binding changes to the backstop would unlock support.
Critics of May???s deal claim that the backstop, which would keep the UK in a customs union to avoid a hard border on the island of Ireland if there was no alternative, could trap the UK indefinitely in the EU???s orbit.
The former foreign secretary, Boris Johnson, claimed on Monday that this would leave the UK as ???colony???.
But Barnier suggested he would offer Barclay little succour at the two men???s first meeting, which comes 46 days before the UK???s expected withdrawal from the EU.
???Tonight I will repeat the EU???s positions. I will listen to what the secretary of state has to tell us concerning the alternative arrangements which the UK would like,??? Barnier said. ???But it???s not more than a concept today. I will also evaluate the interest from the UK side for possible changes to the political declaration, which, let me remind you, fixes the outline quite precisely for the future separation.???
The withdrawal agreement and accompanying political declaration agreed by the British government and the EU was rejected in the Commons by a historic margin last month, leaving May with a mountain to climb if she is to get a deal through.
Barnier said Jeremy Corbyn???s plan to revive the talks by pivoting to support a customs union was ???interesting???, but that he was awaiting fresh ideas from London.
Bettel, who described the UK as the ???disunited kingdom???, told reporters: ???Today the response of the Brexiteers is non, non, non et non. But they haven???t actually proposed any alternatives.???
Barnier echoed that call, insisting that the clarity over the next steps ???has to come from London???.
???It???s in London where they have to find the ways and means to build a positive majority between the two negative majorities that exist today in the House of Commons,??? he said.
The EU official said Brussels could offer only further explanation rather than renegotiation.
???Maybe there???s a way to explain better, to have more ambition, to put into perspective the content of the accord, and the backstop???, he said.
???We stand ready to do this work on the substance of the political declaration, but for all the reasons confirmed by the president of the council ??? European commission and parliament, from the European side we consider that the work done on the withdrawal agreement on the organisation of the separation cannot be reopened.??????
Barnier said May had insisted last week that she was not seeking an extension to article 50 that would delay Brexit beyond 29 March. But he admitted that time was ???extremely short???.
Talking at a conference in Munich, the EU???s deputy chief negotiator, Sabine Weyand, suggested there was no chance of Brexit being reversed as there was no majority in parliament for a second referendum.