A £50bn “Brexit bill” that Britain must pay will be “one of the first issues on the table” in the negotiations, Theresa May has been warned.

Michel Barnier, the European Commission chief negotiator, has confirmed the UK will have to pay the fee for outstanding liabilities, EU leaders said.

The sum is believed to include the obligation for the UK to pay into the EU Budget until the end of 2020, as well as pensions liabilities and payments linked to loan guarantees.

The Czech Republic's Europe minister said the issue would be near the top of the list when the exit talks get underway next year.

Tomas Prouza told Sky News: “This is agreeing the bills that the UK has already agreed to pay.

“We’re talking about payments to the existing budget that the UK already voted for, pensions of British citizens working at the EU.

“This is only things the UK has already committed itself to paying.”

Theresa May left standing on her own at Brussels EU summit

When asked if British people should expect a bill worth tens of billions of pounds, Mr Prouza replied: “Definitely. This is what the UK has already committed to pay, and we would expect that the UK would honour its commitments. It will be one of the first issues coming up on the table.”

Downing Street said the UK would meet its obligations while it remained a member of the EU – but any financial settlement after that would be a matter for negotiation.

“Decisions on how UK taxpayers’ money is spent will be decisions for the UK to take moving forward,” a No 10 source said.

“There’s a whole range of complex issues that will need to be resolved in the negotiations.”

The source added: “One of the issues that will be for discussion and negotiation is the EU budget contribution and things like that.

“The important thing is that, once we have left, the UK Government will make decisions on how taxpayers’ money is spent.”

Mr Barnier has reported a figure of €50-60bn (£42bn-50bn) to EU leaders in his tour of EU capitals.

The same figure was mentioned to Brussels based diplomats from the EU-27, in a meeting last month, Sky was told.

Another analysis of the same liabilities by two EU finance ministries is believed to have calculated the bill at around €40bn (£33bn).

Also on Thursday, Theresa May told European Union leaders that she wants an early agreement on the status of Britons living on the continent and EU citizens in the UK after Brexit.

Irish Taoiseach Enda Kenny, speaking following a European Council summit meeting in Brussels, said Ms May updated her fellow leaders on the Supreme Court case on Article 50 and her hope for a deal on EU nationals.

“She would like to have the question of UK citizens living in Europe and European citizens living in the UK dealt with in the early part of discussions that take place,” he said.