The Frenchman said it was “possible” that the UK could walk away without paying a penny of the EU’s proposed ‘Brexit bill’.
The divorce settlement, estimated to be as high as £88bn (€100bn) is a major source of tension between the two sides.
Last week, he gave UK officials a two-week deadline to clarify how much Britain is willing to pay before it leaves the union.
Discussing the likelihood of the talks breaking down, Mr Barnier said: “It’s not my option, but it’s a possibility.
“Everyone needs to plan for it, member states and businesses alike. We too are preparing for it technically.
“On 29 March 2019, the United Kingdom will become a third country.
“A failure of the negotiations would have consequences on multiple domains.”
The bureaucrat then urged the UK to strike a Norway-style deal, keeping the country in the EU’s single market and customs union.
He said: “The only trade-free assumption would be that the UK remains in the customs union and in the single market. This is an option for us, it’s still possible.
“But the government of Theresa May has preferred to leave the single market and the customs union, and stick only to a trade agreement.”
Mr Barnier also said it was crucial that Brexit does lead to the re-introduction of a hard border between the UK and Ireland.
He told French newspaper Journal du Dimanche: “The tragic conflict between the Protestants and the Catholics was not that long ago.
“I won’t do anything that could weaken the Good Friday Agreement.”
Earlier today, top Brexiteer Michael Gove said was in favour of handing over extra cash to Brussels to secure a good exit deal.
The Environment Secreatary said the Government was doing “everything we can” to secure a positive agrement with the EU.
Asked if he would block the Prime Minister if she wanted to pay a big divorce settlement, he replied: “I certainly would not.
“I would not block the Prime Minister in doing what she believed was right.”
But businessman Sir James Dyson insisted it was time for the PM to walk away from the Brexit negotiations.
The entrepreneur told the BBC: “Demanding billions and billions to leave is quite outrageous and demanding it before we negotiate anything is outrageous.
“So, I would walk away. I think that’s the only way to deal with them.”