THERESA May is completely deluded and underprepared for Brexit, according to leaked details of a disastrous dinner between the Prime Minister and European Commission President Jean-Claude Juncker.

German newspaper, Frankfurter Allgemeine Sonntagszeitung (FAZ) reported yesterday that Juncker left the Downing Street meeting feeling “10 times more sceptical” about Britain’s ability to not make a mess of Brexit. Despite having 11 months, May and her ministers have seemingly not gone far beyond “Brexit means Brexit”.

Number 10 deny the story, saying they don’t recognise the account, and the Commission have declined to comment.

During the dinner last Wednesday, May supposedly told Juncker and his officials that she expects to be re-elected as PM and told them she wanted to make Brexit “a success”.

Juncker replied by saying Britain will now be a “third state”, and not in the customs union, which meant that “Brexit cannot be a success”.

May further surprised EU officials when she suggested the issue of guaranteeing the rights of EU nationals living in the UK and British nationals living in Europe could be sorted at an EU Council meeting at the end of June. This timetable, the newspaper suggests, is far too optimistic given the complexities on negotiations like rights to health care.

At one point Juncker pulled two piles of paper from his bag – Croatia’s EU entry deal and Canada’s free trade deal – to make the point that Brexit will be exceptionally complex.

The EU side felt May was seeing the whole thing through rose-tinted-glasses, and, damningly, that she had not been fully briefed by ministers on what was happening.

Juncker and his officials seemed to think there was a major communications and briefing problem, with important messages from Berlin and Brussels not getting through to the Prime Minister, leaving May uninformed and with startling misconceptions about Brexit and the approach to negotiations by the EU’s remaining 27 countries.

What might worry May more about the report is that it appeared at all. The Prime Minister has always been unwilling to give a “running commentary” on negotiations. The leak to FAZ suggest that view is not going to be reciprocated by the Europeans.

May even asked the Commission to work through the Brexit talks in monthly, four-day blocks, with everything “secret” until the end of the process.

The Commission said it was impossible to reconcile this with the need to get approval from member states and the European Parliament. Documents would need to be published.

May then insisted to Juncker that the UK will not have to pay a divorce bill as there is nothing to that effect in the treaties.

She was told in response that the EU was “not a golf club”.

Brexit secretary Davis Davis is said to have interjected here and told Juncker and his officials the EU could not force the UK to pay the bill.

While Juncker seemed to acknowledge that, he replied that such a move would mean no trade deal for the UK.

The paper also suggests Davis made three separate references during the dinner to having successfully blocked May’s one-time plans for extra powers of surveillance for Britain’s security services via the European Court of Justice.

The newspaper said May was “unhappy” at this intervention and that Juncker’s circle are no longer sure if Davis will still be in charge of negotiations after the election.

The next morning, FAZ reports, Juncker called Merkel on her mobile, and said May is living in another galaxy and totally deluding herself.

Scotland’s Minister for UK Negotiations with the European Union Michael Russell said: “This is a devastating account which lays bare the reality of the Tory Government’s weak and chaotic leadership of Brexit – having seen first-hand how the Tories negotiate on EU matters, I am frankly not surprised by anything reported here.

“Behind the façade and the robotic sound-bites the Tories are quite clearly not being straight with people about their plans for Brexit.

“This looming catastrophe shows precisely why Theresa May mustn’t be allowed to block the decision of the Scottish Parliament to give people in Scotland a choice over their future when the terms of Brexit are clear.”

He added: “It is no wonder that Theresa May is running scared of media scrutiny and so determined to use this election to crush all parliamentary opposition.”

LibDem leader Tim Farron said: “These reports blow a massive hole in the Conservative Party’s arguments. Theresa May chose a divisive hard Brexit, with Labour’s help, and now has no idea what to do next. This government has no plan and no clue and this shows it starkly.”

Shadow Brexit minister Keir Starmer told a newspaper: “This is ... evidence that May’s rigid and complacent approach to Brexit negotiations risks leading Britain over a cliff edge.”

A Downing Street spokesperson said: “As the Prime Minister and Jean-Claude Juncker made clear, this was a constructive meeting ahead of the negotiations”.