British Prime Minister Theresa May on Sunday denied she was "in a different galaxy" after European Union leaders were reportedly dismayed by her Brexit negotiating demands at a meeting this week.

May told European Commission President Jean-Claude Juncker and chief  negotiator Michel Barnier that a detailed potential trade deal needed to be drawn up before Britain would agree to pay its EU divorce bill, according to  the Sunday Times.

According to the paper, Juncker told German Chancellor Angela Merkel that May was "in a different galaxy," adding that it looked more likely now that no  deal would be reached at all.

May told BBC's Andrew Marr Show that she stood by her earlier comment that "no deal was better than a bad deal", and rejected claims that her negotiating stance was unreasonable.

"I'm not in a different galaxy. What this shows is that there are going to be times when these negotiations are going to be tough," she said.

"I want to ensure we agree on a trade deal and withdrawal arrangements  for...when we leave the European Union.

"And the EU has itself said that nothing is agreed until everything is  agreed," she added, suggesting that further battles loom over the structure of  the negotiations.

Luxembourg Prime Minister Xavier Bettel said the EU estimated the bill to  be 40-60 billion euros ($42-65 billion), which mainly covers financial  commitments made by the bloc while Britain was a member.

EU leaders unanimously backed a tough Brexit strategy at a summit on

Saturday, demanding a "serious response" from Britain on the rights of European  citizens before trade talks can start.

The summit was the first since May one month ago formally triggered the  two-year process of untangling Britain from the EU after four decades of  membership.

May said there was agreement that the fate of EU nationals living in Britain should be an early priority, but that as prime minister she also had "a care for the UK citizens living in the other 27 countries of the EU," calling for a reciprocal deal.

"There is goodwill there," she explained. "I believe we can give that  assurance to those people at an early stage."

The prime minister surprised the political establishment earlier this month by announcing a snap general election for June 8, calling it on Sunday the  "most important election our country has faced in our lifetime."

May is expected to massively increase her Conservative Party's slim majority, saying that would help her "take a strong hand into negotiations".

Actual Brexit negotiations are not expected to begin until after the  British election, although the EU is set to give an official mandate to Barnier on May 22. (AFP)