He claimed there were still "problems" in Brussels "understanding the position" of the British Government. The EU has warned that Single Market access is out of the question if London decides to restrict the ability of its citizens to live and work in Britain.
The EU Chief Negotiator explained that if the Conservative government's position had changed from the previous guarantee of some sort of customs union like harmonizing that would guarantee no new hard border across Ireland, then a hard border with customs checks would be "unavoidable" along the hundreds of miles of countryside between Northern Ireland and the Republic.
"There is little time, and we don't have a minute to lose if we want to succeed", Barnier told reporters in Brussels. She also said there would be no difference in the rules in the North as in the rest of the UK.
"If these disagreements persist the transition is not a given".
Analysts say that avoiding a physical border - along with related "checks and controls" - means that Britain must remain very close to the EU.
London has already pledged to avoid the return of a hard border on the island of Ireland, an extremely complicated issue that was not fully resolved in the first phase of negotiations.
He added that the forecasts were "a work in progress" and said: "You wouldn't drive a auto that's half finished; you wouldn't use a forecast that's half completed".
"This would be achieved by agreement of the parties to interpret relevant terms in these global agreements, such as "European Union" or "EU Member State", to include the United Kingdom", it said.
But the British currency quickly reversed those gains on Friday after the EU's Michel Barnier said London had "substantial" objections to the EU's transition offer and that parts of it were not up for negotiation. "The hope must be for a deal which allows us to visit the EU and European motorists to come both as easily as possible", he added.
Plaid Cymru's Brexit spokesman, Hywel Williams, said: "If Northern Ireland stays within the EU's single market and customs union, it becomes an economic necessity for Wales to be given the same deal".
He said he warned his British counterpart, David Davis, this week that "the moment has come to take decisions".
The Japanese ambassador to the UK, Koji Tsuruoka, said afterwards: "Japanese companies are enjoying their operations in the UK and they have also today said they would very much like to continue this successful operation in the UK, which of course is comprised with access to the European market".
Mr Davis said that "for any such period to work, both sides will need a way to resolve disputes in the unlikely event that they occur". Yet at the same time they dismissed the UK's push for reasonable safeguards to ensure our interests are protected.
A spokesman for the government said earlier this week that it believes it can achieve "as frictionless as possible" trade with the European Union because doing so is in the interest of both parties.