Furious Conservative MPs have urged Theresa May to stand firm against an EU threat to take Britain to the International Court of Justice to enforce a £50bn Brexit “divorce bill”.

A draft plan – obtained by a Dutch newspaper – revealed the EU is readying for a long legal battle at The Hague if Britain tries to walk away without meeting its huge liabilities.

“In that case it is: see you in The Hague!” it quoted an EU official – in response to the Prime Minister’s vow to leave with “no deal” if necessary, perhaps seeking to avoid any exit bill.

Neither Downing Street nor the European Commission denied the prospect of a court fight if the negotiations break down, both declining to comment on a leaked document.

A Government source told The Independent that it recognised the divorce bill as among a “wide range of issues for the UK and the EU that will need to be addressed as we leave the EU and agree a new partnership”.

But the tough EU stance sparked an angry response from Brexit-backing Tory MPs who insisted it was an empty threat which Ms May could easily ignore.

It came as Spain’s deputy minister for European affairs insisted the UK must agree in principle to pay the exit bill before any talks on a trade agreement can begin.

Theresa May pledges prosperity to whole of UK

John Redwood, the former Cabinet minister, said: “There is no case to answer on the so-called divorce bill. It is not a divorce.

“The UK is leaving a treaty organisation under the rules of that organisation. The treaty makes no provision for charging an exit fee.”

A second ex-Cabinet minister, Iain Duncan Smith, said: “It’s nonsense the UK owes the EU any money. The EU Commission has to calculate the value of its assets because, when it does, it will be clear that, contrary to what some are trying to claim, the EU owes the UK money - not the other way round.”

Jacob Rees Mogg dismissed the idea of any case reaching The Hague, which simply existed for arbitration and was “not a court as we know it”.

“This is one of those things said by people who have not bothered to read the legal situation,” he said.

And Sir Bill Cash – who has pointed to the UK helping Germany waiving half of its war debt in the 1950s – said: “I don’t think the International Court has any jurisdiction in this matter.

“I have no doubt that there will be all kinds of the usual academic suspects running around saying we should do this, that or the other. There is no reason for us to make any payment.”

But Tim Farron, the Liberal Democrat leader, said the prospect of the fight ending up at the International Court exposed Ms May’s position as “utterly risible”.

“The Government is going to have to go to The Hague to try to fight a Brexit bill that they are forcing on us,” Mr Farron said. “They have made a choice to pursue a hard Brexit and we will all the price.”

A lengthy battle at the International Court would hold up attempts to reach a new trade agreement with the EU, if it insists on settling the controversy over money owed first.

The leak, published by the respected de Volkskrant newspaper, said the EU strategy would also:

* Insist access to the EU single market depends upon the UK accepting the "four freedoms" – including, crucially, freedom of movement.

* Propose a deal guaranteeing both the future rights of EU nationals in the UK and Britons in EU countries.

* Demand that the UK loses some of its existing trade advantages, as the price of leaving.

The newspaper billed its story as “the secret EU Strategy for separation from the British”, based on information provided by key EU insiders.

The EU’s apparent strategy was published shortly after Mr Tusk announced that EU leaders will meet to agree their strategy at a special summit on 29 April.

Significantly, he vowed to make "the process of divorce the least painful for the EU" – without mentioning what pain may lie ahead for Britain.