The European Union's chief Brexit negotiator says that Britain should have less than two years as a transition period after it leaves the European Union and that the U.K. would have to live by all the key EU principles.

Michel Barnier said that the "logical end" of a transition period must be Dec. 31, 2020, giving Britain enough time to adapt to its situation outside the EU and not too long to create further problems.

Barnier said Wednesday that "there will be no transition a la carte. All the rules and policies of the EU must continue to be applied."

Any full decision will still have to be negotiated between Barnier and British counterpart David Davis. The United Kingdom is set to leave the EU on March 29, 2019.