Reports based on Brussels and Whitehall sources said European diplomats were becoming increasingly concerned that his “aggressive” tactics will backfire on the EU.
In a sign of growing splits among the governments of the 27 nations staying in the bloc after Brexit, some are understood to be urging Mr Barnier to turn down his rhetoric.
Hints at European frustration with the negotiation process followed a series of threats from the veteran diplomat last week.
Brussels documents called for the EU to get summary powers to punish the UK during the expected two-year transition out of the bloc.
And in an outburst on Friday, Mr Barnier claimed the transition was "not a given".
One EU diplomat was yesterday quoted in a Sunday newspaper as saying: "Could anyone accept these terms? If I was Britain I would be tempted to say 'no', walk away and then see how the EU does without the money."
Tory Cabinet minister Penny Mordaunt yesterday acknowledged that other EU countries were getting frustrated with the European Commission's approach to the Brexit talks.
"Other nations involved in this are very pragmatic and have not been impressed with some of the language that the commission has used," said Ms Mordaunt, the International Development Secretary.
Tory MP Daniel Kawczynski, chairman of the Commons All-Party Parliamentary Group on Poland, said: "In private, many Polish politicians are expressing regret at the commission not treating Britain with due consideration and respect."
A Whitehall source was quoted in a Sunday newspapers as saying French diplomats were "cross at the lack of consultation" before the publication of the Brussels document about the transition process.
The source went on to describe the document as "an aggressive political opening shot".
Meanwhile, Mrs May was coming under fresh pressure from Brexit campaigners last night to reject Brussels demands for restrictions on the Government's freedom to negotiate new trade deals around the world during the transition period.
Change Britain, a pressure group founded by senior Tory Michael Gove and other Leave campaigners after the EU referendum in 2016, said a clause buried in the negotiation document insisting the UK must be bound by "sincere cooperation" during the two-year period effectively put a ban on trade negotiations.
Gisela Stuart, chairman of Change Britain and a former Labour MP, said: "The EU’s current proposal includes a devious clause that will tie the hands of the UK, and threatens to leave us in perpetual transition.
"It’s vital that the UK is given the freedom to properly prepare for Brexit, allowing businesses time to adapt to our future trading arrangements.
"The Government must now wake up to the EU’s tactics, take action and make clear decisions. If they fail, they risk leaving the UK in an unacceptable state of legal limbo."