The president of the European Parliament has urged Britain to enact Brexit as soon as possible, ahead of a meeting with Theresa May.
Martin Schulz said he wants the "earliest possible triggering of Article 50" and that negotiations for Britain's departure should finish before European elections, which are scheduled for mid 2019.
Article 50 starts a two-year countdown on negotiations, so it would need to be invoked by May next year to fit with this timetable.
In a statement issued ahead of his meeting with Mrs May at Downing Street, Mr Schulz said: "The future deal between the EU and the UK must be good for all sides and must be one which allows the UK and the EU to keep working closely together on various aspects.
"The UK has decided to leave the EU but it will continue to be a European country with values and geopolitical interests which are common to the rest of the continent.
"I will also reiterate that the four freedoms of the single market - goods, capital, services and persons - are equally important.
"In London I will also stress why the European Parliament favours the earliest possible triggering of Article 50, which is a pre-condition to opening negotiations."
Mr Schulz, a member of Germany's centre-left Social Democrats, who leads the 28-nation EU's legislative assembly, said he "understands that the British government wants to take its time".
But the dpa news agency reported that he warned it would not be good for Britain or the EU if the British were voting for members of the EU parliament while negotiations to leave the bloc were still going on.
Mr Schulz is in Britain for a two-day trip where he will hold a series of talks on Brexit.
After his meeting with the Prime Minister he will meet the mayor of London Sadiq Khan on Thursday.
He will then meet Jeremy Corbyn on Friday before going on to give a talk at the London School of Economics called The EU and the UK - parting ways but working together.
Speaking ahead of his meeting with Mr Schulz, Mr Khan said: "I am sure that the president already realises that although the decision to leave the EU was not what the capital wanted, we look forward to working closely with the Union and its member states to boost economic prosperity both here in London and across the continent."