The civil service is not ready for the enormous task of driving through Brexit, former Cabinet Secretary Gus O’Donnell has warned.

The former head of the civil service added his voice to the growing number of former Whitehall leaders and organisations highlighting the daunting problems ahead.

Lord O’Donnell also echoed criticisms that Brexit will “crowd out” the ability of the civil service to perform other key task, after years of severe cuts – without a big recruitment drive.

Asked, by The House magazine, if the civil service is prepared, Sir Gus replied: “There’s a very simple, short answer to that - which is no.

“Brexit imposes a lot of extra requirements on the civil service. They’re not perfectly ready.”

Lord O’Donnell said the civil service was “gearing themselves up for it”, but added: “It will mean bringing in new people, developing the skills in all sorts of areas and expanding them into other areas.

“I’m confident that they will get there. But no-one should be under illusions – this is an enormous job.”

The Crossbench peer argued every Government department – not simply the three directly dealing with Brexit – would need to be beefed up for the task ahead.

And he added: “What I worry about is it will crowd out other things, the whole modernization.

“I hope that those things won’t stop by everyone having to spend their time doing negotiations on our relationship with the EU, the world outside the EU, immigration policies, ECJ stuff, all of that, all huge. It will last for years and years.

“Now there are options there. One is you bring in more people, the other is you stop doing some of things that you’re doing at the minute.

“And I think government will have to make up its mind which of those it will have to do.”

The stark warning comes hard on the heels of the chief executive of the civil service arguing Whitehall is trying to do 30 per cent too much.

John Manzoni said ministers would need to scale back their ambitions or risk future policy failures.

And Sir Simon Fraser, a former permanent secretary at the Foreign Office, warned an influx of new civil servants was needed to handle the Brexit negotiations and their fallout.

“The government is still in information-gathering mode and is not yet at the point of integrating that into a central plan,” he told the Brexit select committee.

Also, this week, the Institute for Government (IfG) described Brexit as an “existential threat” to cash-starved Government departments, with the process appearing “chaotic and dysfunctional”.