The source said that a Leave vote will simply see the EU pull in new member states, something the Brexit campaign has consistently warned will happen if Britain votes to stay in the EU.

“The EU will have found an identity and will have moved forward, deepening in key areas like monetary union and defence,” the source said. The source dismissed Britain as “not a player” in core areas of the EU and said that people in Brussels now believe it will “be better to have a reluctant player outside the tent”.

“Britain already sits at the table and in most core areas, you are not a player,” the source said. “In other areas we wish you were a player, but you are hardly present. Some might say it will be better to have a reluctant player outside the tent.

“For Europe, British membership of the EU is really a 50-50 advantage-disadvantage question.” In an attack on David Cameron, the source said that the Prime Minister made a “big mistake” thinking he could change the minds of British voters after his party spent 20 years “demonising Brussels and the EU”.