Brusells: EU Brexit negotiators have been left "flabbergasted" after their British counterparts launched a legal deconstruction of the so-called "Brexit bill", as Brussels talks to resolve it headed for an increasingly acrimonious impasse, EU sources said.

British negotiators spent three hours launching a painstaking, line-by-line rebuttal of the EU's demands for a €100 billion ($150 billion) divorce settlement to the barely concealed fury of EU negotiators.

"There was total amazement," the EU source said, "Everyone was completely flabbergasted that this young man from Whitehall was saying that the EU's preparation on the financial settlement was 'inadequate'. It did not go down well."

The "intense" exchanges were the first time the two sides have butted heads over the details of the divorce bill and sets up what was expected to be a frosty joint press conference between David Davis, the Brexit Secretary and Michel Barnier, the EU chief negotiator. Mr Barnier again accused the UK of failing to specify its positions.

"To be flexible you need two points, our point and their point," he said. "We need to know their position and then I can be flexible."

British negotiators have consistently refused to say what the UK is prepared to pay for Brexit, but have sought to turn the tables on the EU side by attacking the legal basis for their demands.

In an 11-page presentation containing 23 slides and 47 paragraphs of dense text, the British negotiators argued that the European Commission's four-page position paper on the financial settlement provided scant basis for a settlement.

"The UK is not convinced that the European Commission's paper [on the financial settlement] is satisfactory," said a source with knowledge of the UK strategy.

Among the slides was a reference to the key part of Article 50 which says a member state's withdrawal from the EU should be agreed with reference to the "future relationship".

The EU side is refusing to discuss a future EU-UK relationship until there is "sufficient progress" on the issues of the financial settlement, citizens' rights and the Northern Irish border - all issues the UK says cannot be settled without reference to a future deal

With the impasse over the talks deepening, Mr Barnier and other top EU officials have warned that the chances of Britain progressing onto future relationship talks in October, as originally planned, are now receding.

Theresa May, on a trip to Japan, told the EU "significant discussions" had to take place before the size of the so-called "divorce bill" could be agreed.

Telegraph, London