Pierre Moscovici, the EU Commissioner for Economic and Monetary Union, made the dire warning for Brussels as the bloc has to make up a funding shortfall of £9billion after Brexit and member states have been called on to cough up the cash.
Speaking in an interview with French TV station C News, Europe 1 radio and newspaper Les Echos, former French finance minister Mr Moscovici said he had spoken to Tony Blair at the World Economic Forum in Davos.
He said Brexit was a “lose-lose” situation as it is a loss for the UK and also for Europe.
He said: “Losing a state that is a permanent member of the United Nations Security Council, which is a great economic partner, creates a whole series of problems.”
During the interview, he also said: “The door is open if the British people want to change their mind, very good.
“But I want to show a bit of scepticism, the British have voted, I regret this vote but I also respect it”.
He said he is sorry about the Brexit vote but it is now time for the negotiations to ensure the UK can have the most friendly and close relationship possible with the EU.
The Commissioner said: “The UK can’t have their cake and eat it, we must now negotiate an agreement that allows us to have the closest relationship possible with the UK.
“Being outside the EU is not inside, they can’t have the advantages of the single market and the customs union.”
The European Commission President Mr Jean-Claude Juncker has also said he would be happy to help Britain to rejoin if it had a “change of heart”.
He said earlier this month: “We are not throwing the British out, we would like the British to stay, and if they so wish, they should be allowed to do so”.