The European council president, Donald Tusk, has called on Theresa May to show “moderation and respect” in the increasingly heated Brexit negotiations, a day after the prime minister launched an extraordinary broadside accusing Brussels of trying to meddle in the UK election.
Tusk warned that talks over the terms of Britain’s exit would fail before they even began if emotions continued to run wild.
Appealing for a ceasefire, he also appeared to chide the team around the European commission president, Jean-Claude Juncker, for leaking an account of a dinner in Downing Street where the two sides clashed over Britain’s divorce bill.
“These negotiations are difficult enough as they are,” he told reporters at a press conference in Brussels.
“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.”
Standing next to the Norwegian prime minister, Erna Solberg, with whom he had discussed Brexit, Tusk added wryly: “At times like this, it is more important than ever to have a strong and close partner like Norway.”
Norway is not a member state of the EU but is a member of the European Economic Area (EEA).
May delivered an unexpected broadside against the EU on Wednesday afternoon, claiming the European commission and unnamed officials had been trying through various means to meddle in the UK election campaign.
“Britain’s negotiating position in Europe has been misrepresented in the continental press,” she said in a statement outside No 10.
“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 which will take place on 8 June.”
Within minutes of Tusk’s intervention, however, senior Tory sources suggested the comments could be aimed as much at Juncker, whose account of the apparently frosty dinner in Downing Street was leaked to the German press, as at the prime minister.
The UK source said Tusk’s hopes that participants in the talks would display “discretion, moderation and mutual respect” described their own approach to the negotiations.
May’s comments had been dismissed by many in Brussels as cynical electioneering, but Tusk’s public slapdown reveals his growing fear that the coming talks could become poisoned beyond saving.
The former deputy prime minister, Nick Clegg, said: “Blaming foreigners and an unsubstantiated European plot for her own government’s shortcomings is more worthy of [Turkish president Recep Tayyip] Erdoğan than Downing Street.”
Conservative strategists believe May’s bellicose performance outside No 10 will have played well with voters who are keen to see Britain take an assertive approach to the talks.
The Tories hope to perform well in the Labour heartland seats on 8 June, partly by winning over voters who backed Ukip in the 2015 general election.
The Brexit secretary, David Davis, upped the ante later on Thursday night, saying on the BBC1’s Question Time that the European commission had tried to bully the British people.
“There was a dinner, I was at the dinner, and what came out afterwards was not a leak, it was a deliberately misleading briefing, to position the commission in one position and to undermine the position of the British government,” he said.
“The response to that by the government, by the prime minister was, we simply said: ‘We don’t recognise this.’ That’s all, it was very polite, and for 48 hours we stuck to that.
“Then we had further briefing – we’re going to have to pay €100bn [£85bn]; we’re going to hear the prime minister will not be able to negotiate – and eventually it got to the point where a line was crossed.
“Clearly what was happening was the commission was trying to bully the British people, and the British people will not be bullied, and the government will not allow that.”
Backing the prime minister’s insistence that she would take a combative approach to the talks with the rest of the EU, he added: “We’re very lucky we’ve got a bloody difficult woman.”
The shadow business secretary, Rebecca Long-Bailey, also appearing on Question Time, said the government should take a more emollient tone in order to get the best Brexit deal.
“Ultimately what we need now is a government that puts collaboration and patriotism at the heart of our Brexit negotiations so that they get a deal for the many, not the few.”
However, the European commission’s chief spokesman, Margaritis Schinas, also dismissed May’s allegations as excitable electioneering earlier on Thursday. “We are not naive,” Schinas said.
“At the moment, there’s an election taking place in the UK. People get excited when we have elections. The election in the UK is mainly about Brexit.
“We here in Brussels are very busy, rather busy, with our policy work. We have enough on our plate.”
The president of the European parliament, Antonio Tajani, also weighed in to dispute the prime minister’s claim.
“No one is trying to influence the outcome of the election campaign in the United Kingdom,” he said.
“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.
“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.”
Senior diplomats representing the 27 member states that will remain once the UK leaves in 2019 voiced their concerns that the rhetoric could go too far.
“We are going our own ways at the end, but we need to get through this together,” said one.
“Brussels gets used in domestic political debates, but there is no hidden agenda here. The EU’s negotiating guidelines are there to ensure the very best outcome for everyone.”
A second EU source intimately involved in the drafting of the EU’s position also dismissed the claim as electioneering, telling the Guardian: “Domestic politics is domestic politics and we understand what she is doing. There wasn’t any toughening of negotiating guidelines in that way at all.
“We’ve heard quite a few statements like this and I am quite sure this will not be the criteria the British people use to decide how to vote. The EU negotiators are working seriously and assiduously. They want to be ready for when the UK is ready.”
The chancellor, Philip Hammond, however, continued with the British government’s combative approach in his response to the commission’s announcement on Thursday that it was reviewing the oversight of the euro clearing houses in the City of London, which employ more than 100,000 people.
A number of member states and senior Brussels figures want to force the clearing houses to relocate to an EU member state.
Hammond insisted that the issue would be within the Brexit negotiations, contrary to expectations in Brussels, and that the EU “should be careful of any proposals which might disrupt growth, raise the cost of investment in Europe and the UK or weaken financial stability”.
He said: “London is the world’s number one financial centre, with high standards of financial supervision, including longstanding cooperation with EU institutions.
“This benefits the entire continent. We trust everyone in the negotiations will see the value in not undermining that. These are issues for the negotiations, which will of course be tough at times.”