The European Union's chief Brexit negotiator, Michel Barnier, has said that a deal on the United Kingdom's departure from the bloc must be in place by October 2018.

"All in all there will be less than 18-months to negotiate, once again, that is short. Should the UK notify the Council by the end of March 2017, as (British) Prime Minister Theresa May said she would, it is safe to say that negotiation could start a few weeks later and an Article 50 be reached by October 2018," he said.

In his first press conference since taking up the post two months ago, Barnier said after a deal was agreed upon, it would take months to have it ratified by Britain and the other 27 EU member states. However, he said it's likely the country could be out of the Union before the next EU parliamentary election due in May 2019.

Barnier also said it's still too early to tell if a transition period would be agreed on after Brexit, to allow time for negotiations on a future UK-EU relationship.