Critics warn of '˜policy vacuum' amid claims of '˜major' Brexit speech

Leading civil servants have also suggested his successor will be faced with a “policy vacuum”, as European leaders warn the UK is facing a “hard Brexit” due to a lack of negotiating expertise.

The fall-out from Sir Ivan’s surprise departure came as Number 10 confirmed Foreign Office director Sir Tim Barrow as his replacement in Brussels.

Responding to concerns that the outgoing diplomat was too pessimistic about Brexit, officials were keen to paint Sir Tim as a “seasoned and tough negotiator” who would make a success of negotiations.

It also came amid reports that Theresa May is preparing to deliver a “major” speech in which she will set out her vision for Brexit.

The address is expected to include a commitment to withdrawing from the single market if the EU fails to make concessions on freedom of movement.

This has been interpreted as an attempt to downplay suggestions made by Sir Ivan that the Government is “muddled” in its approach to negotiations and guilty of “ill-founded arguments”.

But Lib Dem leader Tim Farron dismissed the speech as a “ploy” that will end up “on the scrap heap of history”.

“This imaginary speech has been briefed out to try and distract attention from the shambles of this week,” he said.

“Sir Ivan Rogers quit bemoaning the lack of a plan, so rolling out a ‘vision’ less than 48 hours later is farcical.

“Not only is it impossible to deliver what Theresa May claims to be setting out, more importantly it is not in Britain’s national interest.

“The Prime Minister’s mismanagement of Brexit has given her a weak hand, which she is playing badly.”

Minsters and civil servants leapt to Mrs May’s defence, as they praised her choice of Sir Tim Barrow as “just the man” to secure the best deal for the UK.

Brexit Secretary David Davis said Sir Tim would “hit the ground running”, adding: “I am confident that, with his help, the UK will be able to forge a new relationship with the EU that works to the mutual benefit of both sides.”

However, the former diplomat and European Commission official Sir Robert Cooper claimed the new ambassador would be faced by a “policy vacuum” when he arrives in Brussels next week.

While the Norwegian Prime Minister Erna Solberg warned the UK will struggle to cut a deal on its own because it is too reliant on acting collectively as a member of the EU.