Speaking in Brussels, Belgium, Barnier said the process would need to be completed by October 2018 to allow for ratification by the 27 remaining member states within the two-year time scale prescribed by EU rules.
"Should the UK notify the council by the end of March 2017, as Prime Minister Theresa May said she would, it is safe to say negotiations could start a few weeks later and an Article 50 agreement
be reached by October 2018," he said.
Barnier warned the time available to conclude a deal would be short. "All in all, there will be less than 18 months to negotiate."
Barnier stressed that the EU was prepared for the task ahead: "We are ready. Keep calm and negotiate," he said, referring to the British wartime mantra of "Keep calm and carry on."
'Cherry-picking' is not an option
Barnier warned Britain there would be no "cherry-picking" on EU principles, such as access to the single market and freedom of movement.
"Membership of the EU comes with rights and benefits," he said. "Third countries can never have the same rights and benefits since they are not subject to same obligations."
When asked whether this would be a "hard or soft Brexit" -- referring to a decisive cut or a gentler easing of relations -- Barnier was adamant there was only one type of divorce.
"Frankly, I do not know what a hard or soft Brexit (is)," he said. "I can only say what a Brexit is."
Speaking at a summit in Bahrain on Tuesday, May said she was planning on a "red, white and blue Brexit."
"That is the right deal for the United Kingdom," she added.
Court challenge throws wrench in the works
Barnier said there would be no negotiations until the UK had triggered Article 50, formally signaling its intention to leave the EU.
In the meantime, he was building a team of 30 people to ensure the EU was prepared for an Article 50 declaration by the end of March.
For Britain, that timeline may be easier said than done.
May previously planned on triggering Article 50 by the end of March. But the process was thrown into confusion in November when the High Court sided with campaigners who argued the government must seek the support of Parliament first.
The UK Supreme Court has now begun hearing a government appeal
over whether members of Parliament must approve the decision to trigger Article 50.
All 11 judges will hear the four-day case -- for the first time since the Supreme Court was established in 2009 -- before giving a ruling in early 2017.
Barnier's formidable negotiating team
While the British government deals with court challenges, the EU has been assembling a formidable negotiating team for Brexit.
A former French foreign minister, the chief negotiator has already embarked on a tour of all the remaining 27 EU states, diving into the backroom talks
necessary to keep the EU united and in line before Article 50 is invoked.
Barnier has also installed Sabine Weyand -- a senior German negotiator and seasoned veteran of many EU trade deals -- as his deputy.
There were rumors that Barnier would demand Brexit negotiations in French, not English.
But he batted away speculation by tweeting
that he speaks both languages fluently and the official language of talks would be determined at the beginning of negotiations.
Despite his tough talk Tuesday, Barnier has also shown he is not beyond teasing his British counterparts, recently tweeting a photo
of himself in front of the Museum of Broken Relationships