European Commission president Jean-Claude Juncker has said the Brexit process will take "longer than we initially thought", blaming delays on Britain's failure to settle its financial obligations.
Speaking to students at the University of Luxembourg, Mr Juncker said the nations of Europe should be grateful for what Britain had done "during war after war", but added: "Now they have to pay."
Mr Juncker was speaking after the EU's chief Brexit negotiator, Michel Barnier, said talks on issues including the "divorce bill" had not made sufficient progress for him to be able to recommend moving on to the second phase of negotiations, covering trade.
Mr Barnier said the financial settlement had not even been the subject of negotiations during four days of talks this week because the UK was not prepared to indicate how much it was willing to pay.
However a leaked document seen by reporters in Brussels suggested that EU leaders meeting at the European Council summit next week could authorise "internal preparatory discussions" on the shape of a future trade relationship and a transition deal, in a move which could offer British Prime Minister Theresa May hope for talks by the end of the year.
Mr Juncker said: "The first to be impressed by the enormous disadvantages that Brexit ... is entailing are the British. They are discovering, as we are, day after day new problems. That is the reason why this process will take longer than initially thought.
"We had the idea that we would clear all the questions related to the divorce. It is not possible."
On the question of Britain's "divorce bill" - which he previously suggested could come to around €55bn (£50bn) - Mr Juncker said: "We can't find for the time being a real compromise as far as the remaining financial commitments of the UK are concerned.
"As we are not able to do this, we will not be able to say during the European Council in October that now we can move to the second phase of the negotiation, which means the shaping of the Britain/Europe future.
"If you are sitting in a bar and ordering 28 beers and then suddenly one of your colleagues is leaving and is not paying, that is not feasible. They have to pay.
"They have to pay, not in an impossible way - I am not in a revenge mood, I'm not hating the British.
"Europeans have to be grateful for so many things that Britain has brought to Europe, during war after war, before, everywhere and every time. But now they have to pay."