The UK will not enter into “a briefing war” with the European Commission over Brexit talks, Tory sources have said.
It follows reports in a German paper of repeated clashes between Theresa May and Commission president Jean-Claude Juncker at a Downing Street dinner.
EU sources claimed UK misunderstanding of the talks process, and ignorance about how Brussels works, could lead to no deal being agreed on the UK’s exit.
Home Secretary Amber Rudd said the reports were “tittle-tattle”.
She said the emergence of the reports was “not the right way” of negotiating, but the UK was committed to negotiating in “good faith”.
According to the Frankfurter Allgemeine, the prime minister and Mr Juncker reportedly clashed over Mrs May’s desire to make Brexit “a success” and whether the issue of protecting the rights of expat UK and EU nationals could be agreed as early as June.
The newspaper claimed Mr Juncker said: “I leave… 10 times more sceptical than I was before.”
In a speech later on Tuesday, Mrs May – who has dismissed the account as “Brussels gossip” – will cite the need to stand up to the other 27 EU countries.
“We have seen in recent days, it will not be easy,” she will say. “The negotiations ahead will be tough. Across the table from us sit 27 European member states who are united in their determination to do a deal that works for them.”
The German newspaper’s report of the dinner last Wednesday, which looks to have come via European Commission sources, said that after the PM said she wanted to “make Brexit a success”, Mr Juncker’s response was: “Brexit cannot be a success. The more I hear, the more sceptical I become.”
And when she said the UK owes no money to the EU, the president informed her that she was not leaving a “golf club”.
The article said that, after last week’s dinner, Mr Juncker was shocked at Mrs May’s suggestion that a deal on citizens’ rights could be achieved so quickly.
The German newspaper report also suggested Mr Juncker said there would be no trade deal between the UK and the rest of the EU if the UK failed to pay the “divorce” bill which it is expected to be asked for.
Reports also claim that the morning after the dinner last Wednesday Mr Juncker told German chancellor Angela Merkel that Mrs May was “on a different galaxy”.
A senior Conservative source told the BBC that the party would “rise above” all the media coverage but rejected accounts of the dinner, saying: “We really, really do not recognise those reports.”
Ms Rudd said the UK would not be responding to the claims but the government had set out a clear plan and priorities for the talks and Mrs May was the best person to negotiate a Brexit deal that was in the UK’s “national interest”.
“Once you start engaging in gossip, in tittle-tattle in this way, it (will) carry on and who knows where it will lead?” she told BBC Breakfast.
“Nobody knows how much truth there is in gossip. But there are ways of conveying what is going on and this is not the right way.
“I do not recognise the tone in which this has been reported but I come back to the fact that it does make it clear that it is going to be a complex, potentially difficult negotiation at times and who do we want leading those – we want Theresa May leading them, not Jeremy Corbyn.”
Analysis: By BBC Europe editor Katya Adler
Welcome to the EU/UK dominated Brexit Galaxy of Spin and Counter-Spin. A crazy old place. The galactic atmosphere is such these days that the dimensions of truth are elastic; at times, distorted.
Take the arguments this weekend over whether the Downing Street dinner last Wednesday at which Theresa May hosted European Commission President Jean Claude Juncker was a complete disaster or not.
Not at all, insists Downing Street.
But according to an EU diplomat, speaking to Germany’s Frankfurter Allgemeine Zeitung and thereafter hitting Twitter and headlines across the UK, they went “badly, really badly”.
He reportedly went as far as to say the British government was now “living in a different galaxy” to the EU when it came to Brexit expectations. This all seems rather inflammatory, so who’s right and who’s stretching the truth?
The accounts of the dinner were seized upon by European politicians and opposition parties in the UK.
Guy Verhofstadt, the former Belgian prime minister who leads the Alliance of Liberals and Democrats for Europe, said it was time to “get real”. He tweeted: “Any Brexit deal requires a strong and stable understanding of the complex issues involved.”
Labour’s shadow Brexit secretary Keir Starmer said Mrs May’s stance was “wrong, not strong”.
“The PM’s complacent and rigid attitude is isolating Britain and increasing chances of no Brexit deal,” he tweeted.
Lib Dem leader Tim Farron said the backlash in the European media was a “taste of things to come”.
“Whether you see these as leaks or Brussels gossip, the reports show a prime minister who seems to have no idea how difficult these negotiations will be,” he said.
The Green Party, meanwhile, has said voters should be given the chance to change their minds in a second “ratification referendum”.
“Whoever wins this election has a mandate to negotiate on behalf of the British people – but that does not mean that they have a right to impose a final deal,” said its co-leader Caroline Lucas.
UKIP leader Paul Nuttall urged Mrs May not to give any ground, tweeting: “We don’t owe the EU a penny and in fact there is a pretty good case that they owe us.”