Frenchman Barnier added that an interim deal to soften the blow of Britain's departure was "difficult to imagine" unless it quickly told Brussels what it wanted from a Brexit deal.
Despite the tight new timeline Prime Minister Theresa May pledged a "red, white and blue Brexit" following Britain's shock June 23 referendum vote to leave the European Union.
"Time will be short. It's clear that the period of actual negotiations will be shorter than two years," Barnier said in his first news conference since his appointment by European Commission chief Jean-Claude Juncker.
Speaking in a mix of French and English, former finance minister Barnier said the EU was "ready" for May to trigger the official two-year divorce process as promised in March 2017.
But Barnier said that much of that time would be spent getting any deal approved by the remaining 27 EU countries plus the European Parliament and then British MPs.
"All in all there will be less than 18 months to negotiate -- once again that is short," added Barnier, once dubbed the most dangerous man in Europe by a British newspaper when he was the EU's financial services commissioner.
"Should the UK notify the council by the end of March '17 as Prime Minister Theresa May said she would, it is safe to say that negotiations could start a few weeks later and an Article 50 agreement reached by October 2018."
Barnier, who is touring EU capitals to hear their views on Brexit, urged Britain to "keep calm and negotiate", echoing a famously stoic British World War II slogan.
But he also warned Britain that "cherry-picking is not an option" and that it "can never have the same rights and benefits" outside the EU.
Barnier said the EU "needs to know" the full details of Britain's plans for its long-term relationship with the EU before any interim deal was possible.
"As we don't know what the UK wants and is waiting for, it's difficult to imagine a transitional period," he said.
Barnier's tough stance was backed by Eurogroup chief Jeroen Dijsselbloem, who called Tuesday for a "different attitude" from Britain "because the things I have been hearing so far are incompatible with smooth and incompatible with orderly".
But the new timeline seemed to catch the British government unawares.
In London, a spokesman for May admitted Barnier's timetable was new. "It's the first that I have heard of it," the spokesman told reporters.
However British foreign minister Boris Johnson, in Brussels for talks with his NATO counterparts, insisted that "that timeframe seems absolutely ample".
The EU has become increasingly frustrated with May's refusal to set out its demands, with her government hedging its bets between control over immigration and access to the EU's single market.
Speaking during a visit to the Gulf for talks on post-Brexit trade, May insisted on Tuesday she wanted an "ambitious" deal, while still giving few details.
"People talk about the sort of Brexit that there is going to be ? is it hard or soft, is it grey or white. Actually we want a red, white and blue Brexit: that is the right Brexit for the UK, the right deal for the UK," she said.
Responding to criticism of the government's unwillingness to give detail of their Brexit plan, May indicated on Tuesday she would accept demands by opposition Labour MPs to reveal her hand -- as long as they accept her timetable for leaving the EU.
The premier was facing a rebellion by up to 40 lawmakers from her Conservative party over an opposition motion which demanded she publish the details of her Brexit plan before triggering Article 50.
In an amendment to the motion May accepted the demand, on the condition that MPs "respect the wishes" of voters in the Brexit referendum and accept her timetable to launch the exit procedure by the end of March.
The motion and its amendment will be subject to a debate and non-binding vote in House of Commons on Wednesday.
It comes as the government fights a legal challenge at Britain's Supreme Court to stop parliament having the final say on a decision to trigger Article 50, which could delay the whole process.
If it loses the case, the government could use a "one-line act" to try to quickly push it through parliament, James Eadie, a lawyer representing the government, told the court on Tuesday.