"The (Irish) Government's position is clear - we want the negotiations to succeed but this will only be possible with agreement on a legally robust backstop to avoid a hard border in the Withdrawal Agreement, which must apply in all circumstances".
But he also suggested the prime minister needed a fallback plan if her proposal for a temporary backstop was rejected once and for all at Wednesday's summit.
Mrs May won strong support from Cabinet colleagues as she set out two key "sticking points" preventing the conclusion of a withdrawal agreement. Along with France, he said Germany wished for a close relationship with the United Kingdom in the future, working together in trade and in security terms but he also very much mirrored the frustration you find in political circles across the EU.
Some EU leaders, notably President Emmanuel Macron of France which faces questions over intensive cross-Channel trade and traffic, want much more public clarity to show that the bloc could cope with cutting Britain out of EU regulations.
May insisted that original, Northern Irish-only backstop solution needs to be time-limited, something the European Union has not agreed to. Angela Merkel will at least be partially distracted by her own political upheaval at home but everyone including Frau Merkel will be praying this encounter with Theresa May goes rather more smoothly than their last meeting a few weeks ago in Salzburg.
He said: "We have always said that we are happy to show flexibility in terms of how we get to a destination where-by there is a backstop, or an insurance mechanism, in place to reassure people on the island of Ireland that they are not going to see the re-emergence of physical border infrastructure".
Mr Barnier now believes there can be no further progress until EU leaders are allowed to intervene at this evening's European Council summit.
"In presuming to change the constitutional arrangements of the United Kingdom, the European Union is treating us with naked contempt", he wrote in the Telegraph newspaper. He said: 'We have to give [Mrs May] room to negotiate.' He also delivered a specific warning to his predecessor Mr Johnson, saying: 'Boris is a backbencher so... he is free to say exactly as he thinks.
And Mrs Leadsom herself said: "The Prime Minister is doing a very, very complicated job and I'm fully supporting her in getting that done".
"We are all doing our jobs and we are trying to get the best deal for this country, and that's it".
It was that kind of opposition that made it impossible for Dominic Raab, May's Brexit minister, to agree to a deal in Brussels on Sunday.
Ministers arriving for Cabinet on Tuesday morning were offered sausage and bacon sandwiches.
"We sometimes feel the British are negotiating with the British", Belgian Foreign Minister Didier Reynders said, illustrating the EU's frustration.
In Luxembourg, foreign ministers from the remaining 27 European Union states received a briefing from Mr Barnier, who said "more time" was needed to find an agreement that kept the Irish border open. Opponents of Brexit argue a US trade deal means opening up Britain's markets to the likes of chlorine-treated chicken and genetically modified crops.
Germany's Europe minister, Michael Roth, said Berlin's message to Mrs May was: "Take responsibility and be constructive". Belgium's foreign minister Didier Reynders put the chance of a no-deal Brexit at "50/50".