Oliver Robbins, the permanent secretary at the Department for Exiting the European Union, will now be May's EU adviser, the ministry said in a statement.

It said the move would "strengthen cross-government coordination of the next phase of negotiations" in Brussels, which is due to start next week.

Robbins will continue to head the negotiating team.

A source at the ministry told AFP it was a "planned move" and denied a report in the London Evening Standard newspaper that there had been a falling out between Robbins and Brexit Secretary David Davis.

But Keir Starmer, Brexit spokesman for the main opposition Labour Party, said: "Moving key individuals at this critical time adds a whole new dimension to the government's chaotic approach to Brexit."

"Deep divisions in the cabinet and a complete lack of leadership are putting the national interest at risk," he said.

Vince Cable, leader of the pro-EU Liberal Democrats, said: "This is a sign of the chaos and division at the heart of this government".

An official quoted in The Evening Standard said: "Olly has always worried the Brexiteers as to where he is. There were comments going around that he was trying to slow things down."