Leaving the European Union will be "constitutionally incredibly difficult", former cabinet secretary Lord O'Donnell has said, as he expressed optimism about Whitehall's ability to rise to the challenge of taking Britain out of the bloc.
Downing Street on Monday confirmed that a new cross-government civil service unit has been set up under the political leadership of Cabinet Office minister Oliver Letwin, with the new team asked to lay the groundwork for whoever succeeds David Cameron as prime minister.
The team will be made up of key officials from the Cabinet Office, the Treasury, the Foreign Office and the Department for Business, Innovation and Skills, and Cameron has said its job will be to advise on transitional issues and explore options for the UK's future relationship with Europe and the rest of the world. The prime minister said it would be staffed by the "brightest and best" the civil service has to offer.
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CSW is currently awaiting further details from the Cabinet Office on the precise make-up of the team, its funding, and how departmental reporting lines are set to work in practice.
Speaking on Tuesday, Lord O'Donnell – who was the UK's most senior civil servant from 2005 and 2011 – said working for the cross-government team would be "a wonderful job".
And he pointed out that Whitehall has had to respond to many unexpected events in the recent past.
"This is what the civil service do – dealing with crises," he told the BBC. "It's a complicated world. They've done this before, global financial crisis, post-Black Wednesday, all of this."
But he acknowledged that the scale of the EU's involvement in all areas of UK law meant it would be "constitutionally incredibly difficult" and would take "years" to carry out the wishes of British voters, 52% of whom backed withdrawal from the union in last week's historic referendum.
He also highlighted the political uncertainty currently facing the civil service, with Article 50 – which formally triggers two years of talks on a British exit – only set to be invoked once Cameron steps down in the autumn.
"The problem they've got is that we haven't got what 'leave' constitutes. We don't know what leave's all about. We know that everyone would like to have access to the Single Market. We'd like to have more control over the EU parliament. We'd like to have control over migration numbers. So how can you achieve those things which are very, very difficult?"
O'Donnell called for the UK parliament to be given "a big role" in determining the UK's negotiating position. Cameron on Monday said Letwin's team would take representations from MPs on "all sides of the House".
"This is a process that will take some time," the former cabinet secretary said. "There's no question about that, but it's fundamental to our future."