EU officials could hit Britain with a £49billion bill for implementing Brexit
Brussels sources suggested that Michel Barnier, the European Commission's chief Brexit negotiator, fixed the eye-watering sum at a meeting earlier this week to discuss the forthcoming departure negotiations.
He is expected to claim the cash payment is needed to cover redundancy payments and pension liabilities for EU staff set to lose their jobs as a result of Britain's withdrawal as well as to fund already-agreed budget commitments, loan guarantees and spending on EU projects in the UK.
But critics angrily rejected the threat of a swingeing divorce settlement for British taxpayers.
The EU needs to realise that times have changed
Gareth Johnson, Tory MP
Tory MP Gareth Johnson said: "There is no way the EU is going to get that kind of money from us.
"One of the reasons that we are leaving the EU is because people are fed up with paying money to Brussels which is then squandered.
"The EU needs to realise that times have changed. There is not a cat's chance in hell of Britain paying that amount of money."
Ukip MEP Gerard Batten, the EU exit spokesman for the anti-Brussels party, said: "It would be reasonable for the Government to make a proportionate provision for the British civil servants and MEPs who are going to be made redundant and have pensions due to them.
"But there is no way UK taxpayers should be funding the EU after we have left.
"Like anyone facing a drop in income, the EU should reduce its spending when Britain leaves."
Mr Barnier, a French politician and veteran European Commissioner, is set to go head to head with Tory EU Exit Secretary David Davis in negotiations for a departure deal for Britain later this year.
Mr Barnier is set to go head to head with Brexit Secretary David Davis, pictured
Other EU leaders are understood to have been pressing Mr Barnier to demand a massive departure fee to punish the UK for last year's referendum vote to leave the bloc.
French President Francois Hollande and German Chancellor Angela Merkel were believed to have been demanding an even higher divorce bill of more than £60billion.
Other national leaders who are less keen to hurt Britain are thought to have argued for more reduced sum.
Brussels insiders said Mr Barnier agreed 56 billion euros (£49billion) as a compromise at a meeting of representatives of the 27 other EU nations, the European Parliament and the EU Council secretariat.
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Mr Barnier is also set to tell Mr Davis that the British Government cannot expect to negotiate a new trade relationship with the EU until terms of the initial divorce, including the multi-billion pound bill, have been agreed by all parties.
Whitehall sources said Mr Davis was preparing for at least of two weeks of intense talks with Mr Barnier to wrangle over the figure.
Commission officials refused to comment publicly on the figure.
They said technical work was "ongoing" to determine the UK's liabilities and was based on "objective and verifiable" data.
A European Commission spokesman said: "On February 6 2017, Michel Barnier chaired a technical seminar with the EU 27 member states, the European Parliament and the secretariat of the council of the EU.
"The aim of the meeting was to create a common understanding of the key issues that need to be included in an orderly withdrawal agreement.
"It is no surprise that budget experts in the Commission and other EU institutions are making calculations in view of the UK referendum result and the upcoming Article 50 negotiations.
Francois Hollande and Angela Merkel were believed to have been demanding an even higher bill
"Technical work is ongoing and is based on objective and verifiable data.
"We are not discussing numbers publicly in this context."
Discussing the meeting earlier this week, chief Commission spokesman Margaritis Schinas said for Britain to quit the EU without settling its bills would be like leaving a bar without paying for your drinks.
Mr Schinas on Tuesday said: "During the time of its membership, the UK has taken – and probably will take – financial commitments.
"Those commitments should be honoured in full.
"This will be an essential element of the negotiations on the orderly separation.
"It is like going to the pub with 27 friends.
"You order a round of beer, but then you can't leave while the party continues.
"You still need to pay for the round you ordered."
Tory Cabinet minister Liam Fox, the International Trade Secretary, has previously dismissed the idea Britain should pay an exit bill on withdrawal from the EU as "absurd".
The row over the divorce settlement comes just days after MPs voted overwhelming for Mrs May to go ahead with triggering Article 50.
In a historic vote on Wednesday, the Commons voted by 494 to 122 in favour of the Government's European Union (Notification of Withdrawal) Bill.
MPs voted overwhelming for Mrs May to go ahead with triggering Article 50
The measure is due to come before the House of Lords on February 20 after Parliament's half-term break next week.
Richard Tice, co-chair of the anti-Brussels pressure group Leave Means Leave, said: "They are plucking a figure out of the air.
"This is something that needs to be negotiated between Britain and the EU, not some backroom deal done by EU bureaucrats.
"The EU must also take into account the assets Britain has in the EU – it may well be that they owe Britain money."
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A Government spokesman said: "The British people voted to leave the EU and we're currently preparing for a smooth and orderly exit.
"We have been clear all along that we're determined to get the very best deal for the UK and are approaching the negotiations with a spirit of goodwill.
"However, we cannot provide a running commentary on what may be discussed as part of the negotiations."