The UK's "divorce bill" for Brexit will be around £50 billion, European Commission president Jean-Claude Juncker has confirmed.
The EU's most senior civil servant said the precise figure would be "calculated scientifically", but insisted it was not a "punishment" for withdrawal, but merely the settling of commitments made by the UK.
Speaking to the BBC, Mr Juncker described Brexit as "a failure and a tragedy".
He promised that Brussels will approach the negotiation of Britain's withdrawal in a "friendly" and fair way, but warned that European institutions were not "naive" about the process.
He confirmed that the UK will be presented with a bill of around 60 billion euro (£52 billion) after Theresa May formally kicks off withdrawal negotiations under Article 50 of the EU treaties on March 29.
"It is around that," said Mr Juncker. "But that is not the main story. We have to calculate scientifically what the British commitments were and then the bill has to be paid."
The divorce bill covers liabilities for projects which the UK previously agreed to help fund, as well as pensions for EU officials who served during the period of its membership.
Mr Juncker said: "It will be a bill reflecting former commitments by the British Government and by the British Parliament.
"There will be no sanctions, no punishment, nothing of that kind."
Mrs May is due formally to notify Brussels of Britain's intention to leave the EU in a letter to the European Council on March 29 - just four days after the EU's 60th anniversary celebrations in Rome on Saturday.
The letter will set in train a two-year process of negotiation leading to the reduction of the EU from 28 to 27 members on March 29 2019.
Asked how he felt about Brexit, Mr Juncker said: "It is a failure and a tragedy.
"I will be sad, as I was sad when the vote in the referendum took place in Britain. For me, it is a tragedy.
"I am anything but in a hostile mood when it comes to Britain. We will negotiate in a friendly way, a fair way, and we are not naive."
Mr Juncker made clear he places high priority on protecting the status of the three million EU nationals resident in the UK and the one million Britons living on the continent.
"I am strongly committed to preserving the rights of Europeans living in Britain and British people living on the European continent," he said.
"This is not about bargaining, this is about respecting human dignity."
Mr Juncker told the BBC: "Britain has to know - and I suppose the Government does know it - that they have to honour the commitments and the former commitments."
He added: "We are not in a hostile mood when it comes to Britain, because I do think and I do want to have with Britain over the next decades a friendly relationship."
He made clear he did not want other EU nations to follow Britain down the withdrawal route, warning this could be "the end" for the EU project.
"I don't want others to take the same avenue," he said. "Let's suppose for one second that others will leave - two, three, four or five. That would be the end."