Juncker envisages EU of core groups

“We have to come up with an answer to this historical question, which is to ask ourselves: 'Do we want to move ahead as 28 - we’ve already lost the 28th [the UK] - or should it not be that those who want to go forward more rapidly can do so without bothering the others by putting in place a more structured framework that is open to everyone?'”, he said.

Referring to a commission policy paper on the issue due out this month, he added: "I will argue for this in the coming days."

He did not specify in which areas he would like to see core-group integration, but his speech praised the single currency, which comprises 19 out of the 28 EU states.

It also said the EU should press ahead with military cooperation, referring to French and German ideas that were to use an EU treaty clause on “structured permanent cooperation” between a coalition of willing countries.

Speaking to the Bloomberg news agency the same day, Austrian chancellor Christian Kern said the Brexit talks, due to start in March, were about bigger issues than just future UK relations.

“Brexit is not only about the relation to Great Britain, it’s about the role and unity of Europe itself and so therefore it’s really a very sensitive negotiation procedure,” he said.

He predicted the talks would be “not as easy as the British expect” and would cause “frustration” among Leave advocates who had expected a “free lunch”.

He said Britain would face a bill of “around €60 billion” on its EU budgetary commitments and warned against the UK trying to turn itself into a tax haven to compete with Europe.

“If they start to reduce their corporate taxes to an unreasonable level, then there will be a certain reaction,” he said.

"They need access to the European market and so they have to behave in a decent and reasonable manner,” the Austrian leader said.