John Manzoni said the Government must bring in more civil servants to tackle the challenge of preparing new infrastructure and new border controls.

Whitehall departments have taken on 3,000 more staff so far to cope with the UK's EU departure. 

Mr Manzoni told the Public Accounts Committee on Monday: “Do we have the people we need for today? Yes. Do we have the people we need to built all this stuff in 18 months? The answer is no and we’ve got to get them.”

The chief executive said Whitehall has staff for the divorce process of Brexit but was looking for specialists who could run the “implementation phase”. 

He said: “It’s work in progress. We do not have all the people today that we need to build all the things we’ve got to do. 

"You can’t, by the way, just click your fingers and say I need 300 or so people, because you need to decide what you want to do..and then the needs become clear.”

The civil service said 3,750 specialists are needed to deliver Brexit and 2,400 have been found already. 

The experts will include specialists in finance, accounting, law, digital, project management and trade negotiators. 

The size of the civil service has decreased by 20 per cent since 2008 because of spending cuts, from 525,157 to 419,399 at March this year. 

Defra, the agriculture department, has employed more than 400 extra staff to deal with Brexit because 80 per cent of its work is linked to EU regulations.

The business department, BEIS, also needs to recruit hundreds of new staff. 

At a European Council summit in Brussels this week, EU leaders are expected to declare “sufficient progress” in the negotiating phase so that phase one can end and talks can move on to trade.