European Commission President Jean-Claude Juncker on Tuesday warned that Britain would pay a "very hefty" bill for its leaving out of the bloc.
Juncker said the EU will seek payment from Theresa May's government to cover existing spending commitments made by Britain, the Independent reported.
"The British should know this, they know this already, that it will not be at a discount or at zero cost. The British must respect commitments they were involved in making," he told the Belgian Parliament.
"So the bill will be, to put it a bit crudely, very hefty."
Juncker said Britain would need years to negotiate any future trade deals with the EU after quitting the bloc.
"To agree on the future architecture of relations between the United Kingdom and the European Union, we will need years," he added.
Discussions are underway in Brussels on the size of the bill to be presented to the British Prime Minister when she launches withdrawal talks, with several reports suggesting the demand may be as high as 51 billion pounds ($63 billion) over the next six years.
Brussels' position is that withdrawal arrangements – including the "exit bill" – must be agreed before moving on to potentially lengthy negotiations over future trade relations.
The European Commission had told Britain there can be no membership of the European single market without freedom of movement for workers – a key issue for many people who voted for Brexit, the Independent said.
"We need to settle our affairs not with our hearts full of a feeling of hostility, but with the knowledge that the continent owes a lot to the UK," Juncker added.
His comments came during the second day of the House of Lords debate on the European Union (Notification of Withdrawal) Bill.
The legislation, which would allow May to trigger the formal two-year Brexit process, already cleared the House of Commons unamended with big majorities.