The stakes involved in the UK leaving the EU are too high to let “emotions get out of hand”, European Council President Donald Tusk has said.
The warning comes after PM Theresa May accused European politicians of making “threats” against Britain to try to influence the general election result.
She said some in Brussels wanted Brexit negotiations to fail.
At a press conference in Brussels, Mr Tusk said talks would “become impossible” if arguments continued.
“These negotiations are difficult enough as they are,” Mr Tusk told reporters.
“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.”
The call for discretion is thought to be aimed at EC President Jean-Claude Juncker, who reportedly called Mrs May “delusional” following a Downing Street dinner to discuss Brexit.
Mrs May’s stinging attack on the “bureaucrats of Brussels” was delivered from Downing Street on Wednesday afternoon after her meeting with the Queen.
“Britain’s negotiating position in Europe has been misrepresented in the continental press,” she said, referring to a German newspaper’s account of her dinner with Mr Juncker.
“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 8 June.”
Afterwards, Labour leader Jeremy Corbyn accused her of “playing party games with Brexit” and Scottish First Minister Nicola Sturgeon said it was “irresponsible” to “poison” the atmosphere with the EU.
European Parliament President Antonio Tajani rejected the prime minister’s claim and a spokesman for Mr Juncker said his office was too busy to meddle in the UK election.
On Wednesday, a senior EU source told the BBC the PM’s suggestion that officials were trying to affect the election result was “pure fantasy”.
Mrs May’s statement came during a day in which UK Brexit Secretary David Davis and the EU’s chief Brexit negotiator, Michel Barnier, had appeared at odds over the size of the “Brexit bill” the UK would owe when it left the European Union.
Norway’s prime minister Erna Solberg said Brexit was challenging for not only the UK and the EU but for her country too, whose biggest trading partner is the UK.
She said she was “very concerned” about future arrangements between the EU and the UK, but believed it was “possible to find good arrangements in the future”.