Mr Robbins, the Prime Minister's Europe adviser, likes to work behind the scenes. And for many Eurosceptics his influence on Theresa May, the Prime Minister, is total.

And it is this influence over the Prime Minister which is understood to have frustrated David Davis, and ultimately led to the Brexit secretary’s resignation on Sunday evening.

The Daily Telegraph first disclosed rumours that Mr Davis might quit over Mr Robbins’ influence on the Brexit talks last November. Allies of Mr Davis said then that Mrs May had been 'captured' by civil servants including Mr Robbins.

Then earlier this year the BBC’s Newsnight programme quoted one unnamed minister who said he believes that Mr Robbins, 42, regards Brexit as a crisis to be managed rather than an opportunity to be seized.

Mr Robbins’ is able to wield such influence because – to use diplomatic jargon – he is Mrs May’s "sherpa" in the Brexit talks. This means he is at Mrs May’s side during all the key talks, and when decisions are made.

Like Sir Mark Sedwill - the acting Cabinet secretary after Sir Jeremy Heywood stood aside because of ill-health - Mr Robbins is another Whitehall refugee from the Home Office, serving there when Mrs May was Home secretary as its deputy permanent secretary from 2015 to 2016.

Chris Wilkins, Mrs May's former director of strategy, said: "Olly was somebody who had the full trust of the prime minister, had the full trust of Jeremy Heywood as well and was able to own meetings and run meetings in a way that made the process very smooth and very effective."

One diplomat described Mr Robbins as "quite impressive”, adding that his close relationship with Mrs May, who he worked with when she was Home Secretary, was part of his strength.

The only recent blot was when it emerged earlier this month that he left a file containing what were described as “Cabinet-level briefings” on a train from Brussels to London.

Mr Robbins, 43, who is married with three children, followed the classic route of a senior mandarin by studying politics, philosophy and economics at Oxford university in the 1990s.

Friends of the four most recent Prime Ministers - Tony Blair, Gordon Brown, David Cameron and Theresa May - all told the BBC that Mr Robbins stood out among a competitive field of bright young civil servants.