The president of the European Council has warned Theresa May against letting "emotions get out of hand" in a stinging attack after she accused Brussels of seeking to interfere in the Westminster election.
Donald Tusk said Brexit negotiations will "become impossible" if the UK and the EU become embroiled in arguments before the formal talks have even begun.
Speaking in Brussels, he called for discretion, moderation and goodwill to help ease tensions ahead of Britain's exit.
Mr Tusk said: "These negotiations are difficult enough as they are.
"If we start arguing before they even begin they will become impossible.
"The stakes are too high to let our emotions get out of hand because at stake are the daily lives and interests of millions of people on both sides of the channel.
"We must keep in mind that in order to succeed we need today discretion, moderation, mutual respect and a maximum of goodwill."
Mrs May sent shock waves through Brussels with a dramatic Downing Street statement on Wednesday accusing unnamed "European politicians and officials" of issuing threats deliberately timed to affect the June 8 vote.
European Parliament president Antonio Tajani rejected the British prime minister's claim and a spokesman for European Commission president Jean-Claude Juncker said his office was "too busy" to meddle in the election.
Mr Tajani said: "We are not seeking to influence the result in the UK.
"It is better to have an interlocutor who is not constantly looking for votes because they have had the election, in order to work towards a good solution," he said.
"If you have an election campaign, the rhetoric gets sharper and more robust. I don't think there is any question of influencing the campaign."
Earlier, Mr Juncker's spokesman Margaritis Schinas brushed off Mrs May's comments, telling reporters: "We are not naive, we know that there is an election taking place in the United Kingdom. People get excited whenever we have elections.
"This election in the United Kingdom is mainly about Brexit. But we here in Brussels, we are very busy, rather busy, with our policy work.
"We have too much to do on our plate. So, in a nutshell, we are very busy. And we will not Brexitise our work."
The row over alleged Brussels interference in the election blew up after a German newspaper published an apparently well-briefed account of Mrs May's meeting with Mr Juncker and Brexit negotiator Michel Barnier in Downing Street last week, which reportedly ended with the Commission president saying he was "10 times more sceptical" of the likelihood of a successful Brexit.
Speaking outside Number 10 on Wednesday, the PM said: "The events of the last few days have shown that, whatever our wishes and however reasonable the positions of Europe's other leaders, there are some in Brussels who do not want these talks to succeed, who do not want Britain to prosper.
"Britain's negotiating position in Europe has been misrepresented in the continental press. The European Commission's negotiating stance has hardened.
"Threats against Britain have been issued by European politicians and officials.
"All of these acts have been deliberately timed to affect the result of the general election that will take place on June 8."
Norwegian prime minister Erna Solberg said that Brexit was "challenging" not only for the UK and EU but also for her country, whose biggest trading partner is Britain.
"Norway, as a member of the internal market through the EEA (European Economic Area), will be directly affected by the UK's departure from the Union," she said.
"We are of course very concerned about how the arrangements will be in the future between the EU and the UK. We believe it is possible to find good arrangements in the future."
Ms Solberg said she was happy to have received assurances from Mr Tusk and Mr Juncker that the EU will offer an "open door" for Norway – as well as fellow EEA members Iceland and Liechtenstein – to contribute its voice to the Brexit process.
