U.K. lawmakers in Prime Minister Theresa May’s Conservative party hit back at claims from Austrian Chancellor Christian Kern that Britain will be charged 60 billion euros ($63 billion) to leave the European Union as tensions surge ahead of Brexit talks. In a Bloomberg interview on Thursday, Kern became the first EU leader to put a value on the size of the U.K.’s Brexit bill. While May’s office was muted in its public comments, Kern’s warning that there would be “no free lunch” for the U.K. sparked a furious response from senior members of Parliament. “This figure is a nonsense that’s been conjured up by EU officials who are behaving like children,” former cabinet minister Iain Duncan Smith said in an interview. “For the Austrian chancellor to even refer to it is quite absurd. As for saying there’s going to be no free lunch for Britain, we paid so much into the EU budget over the years, we pretty much bought the damned restaurant.” Haggling over the Brexit bill looks like it will mark a testy start to the negotiations once May invokes Article 50 of the Lisbon Treaty, something she has said she’ll do before the end of March. Britain’s Trade Secretary Liam Fox has called the very idea of a charge “absurd” and the government in London is adamant it won’t pay for any EU projects signed after November. While similar estimates have been circulating for months, Kern cited the sum as an EU calculation that will be presented to May once she formally initiates talks. “The check should be around 60 billion euros, that’s what the European Commission has calculated and this will be part of the negotiations,” Kern said in the interview in Vienna on Thursday. “There will be a lengthy debate about the check that has to be paid by the U.K., because 60 billion euros is a significant amount of money.” Craig Mackinlay, vice chair of the European Research Group of euro-skeptic Conservatives, said the EU needs to back up its figures with transparent costings. “I reiterate my demands for a costed and legal analysis of all lines of liability,” he said by phone. “There appears to be a growing consensus within the Commission and now European Chancelleries that the figure is 60 billion Euros, without any apparent basis. I now demand they prove their figures.”