The EU wants to negotiate in a "fair and friendly" way with the UK over Brexit, European Commission president Jean-Claude Juncker has told London Mayor Sadiq Khan.
The Mayor met Mr Juncker during a whirlwind day of talks with EU powerbrokers in Brussels.
A spokeswoman for the commission president said that he conveyed his solidarity with London after the Westminster attack, adding: "President Juncker also listened with interest to the Mayor's views and concerns in the context of the Brexit process.
"He restated that the EU will negotiate in a friendly and fair way."
The remarks came after Mr Khan used a keynote address in Brussels to warn the EU it would be a mistake to "punish" the UK for Brexit.
He said: "Now is the time to be confident in the European Union, and to act with confidence.
"There is no need - as some have suggested - for the EU to send a message, or to instil fear, by punishing the UK.
"Because a proud, optimistic and confident institution does not secure its future by fear.
"My city is not only the beating heart of Britain's economy, but the single most important organ for growth across Europe.
"I say this with friendship and all due respect - but a bad Brexit deal that hurts London would hurt the European Union too."
The intervention, the day before Prime Minister Theresa May is set to invoke Article 50 and formally trigger Brexit, was intended to put the "emotional" argument for a fair deal between the EU and UK.
When asked during a photocall with Mr Khan if he intended to punish Britain in the negotiations, European Parliament Brexit negotiator Guy Verhofstadt said: "Not at all."
After meeting Mr Khan, European Parliament president Antonio Tajani said that the UK could not expect the same benefits as a member state after Brexit.
He said: "Parliament will play a key role in deciding the outcome of the negotiations. And as its president I will ensure that all positions will be fairly represented in this debate.
"We will all play a part towards achieving a fair and orderly divorce between the European Union and the United Kingdom.
"The parliament will ensure that the interests of our citizens will be protected."
Mr Khan's staff said "diary clashes" meant he was not able to meet European Council president Donald Tusk as hoped.
European integration was a project created by the people, for the people. It was a movement of a generation who came together to proclaim 'never again!' after a half-century of horrific war. With the Treaties of Rome on March 25, 1957, the EU's first six members consigned the ghost of Europe's past to the history books, leaving them as a cautionary tale for future generations never to repeat.