The UK's "divorce bill" for Brexit will be around 60 billion euro ($A82 billion), European Commission president Jean-Claude Juncker says.
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 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".
