Jean-Claude Juncker has called Brexit "a failure and a tragedy" in an interview with the BBC.
The European Commission president promised the European Union would approach Britain's upcoming exit negotiations in a "friendly and fair" manner, but confirmed the UK would be presented with a bill for leaving the bloc.
Juncker insisted this wasn't intended as punishment, merely money to settle UK liabilities for existing EU projects.
Reports have suggested the sum could go as high as 60 billion euros (£52 billion), but Juncker didn't confirm this.
He said: "It will reflect former commitments by the British Government and Parliament.
"There will be no sanctions, no punishment, nothing of that kind.
"I will be sad [when negotiations begin]. For me, it is a failure and tragedy.
"I am anything but hostile to Britain. We will negotiate in a friendly, fair way."
His comments come ahead of Theresa May's formal notification of the triggering of Article 50 next Wednesday.