Martin Schulz, European Union President, said on Thursday in Berlin that Britain must leave the EU before parliamentary elections in the bloc in mid-2019.
He said that negotiations on the Brexit could take as longer as two years.
Schulz said that as a result of this, Britain would have to trigger Article 50 of the Lisbon Treaty, which sets the rules for a two-year negotiating process for a nation leaving the EU by the middle of next year.
The EU president said that considering the highly complex modalities of a British exit from the bloc, he could understand that the British government wanted to take its time over the decision.
However, he added that it would be impossible for Britons to take part in the election, which sees member states elect deputies to represent them in the legislature in Brussels, and that Brexit needed to be completed before then.
Recall that a narrow majority of British voters opted to leave the EU in a referendum on June 23.
Prime Minister Theresa May has said she would not trigger Article 50 before the start of 2017.