Story highlights Britain's exit from the EU could be completed as early as October 2018

"Cherry-picking" is not an option, EU Brexit negotiator Michel Barnier warns

(CNN) The European Union's chief Brexit negotiator, Michel Barnier, warned Britain on Tuesday it may only have 18 months to strike an exit deal from the bloc.

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."

"Membership of the EU comes with rights and benefits," EU negotiator Michel Barnier says.

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."