Britain's former top civil servant has warned that it will take "at least five years" to finally conclude the Brexit negotiations.
Lord O'Donnell also said it was "completely blindingly obvious" that some form of transitional arrangement will have to be agreed between Britain and the EU once the two-year Article 50 period ends.
His comments came just hours after it emerged that Britain's ambassador to the EU had warned Downing Street that it could take 10 years to strike a post-Brexit trade deal between the UK and the remaining member states.
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Speaking to The Westminster Hour on Radio 4, Lord O'Donnell – who served as cabinet secretary under Tony Blair, Gordon Brown and David Cameron – said: "We certainly won’t have come to any final arrangements in two years’ time. We might well get to a point where we can symbolically leave but all sorts of details will still remain to be sorted out."
Asked if he agreed with chancellor Philip Hammond, who earlier this week backed a transitional deal between the UK and EU after Brexit, he said: "I think that’s a statement of the completely blindingly obvious. I mean, the idea that you can manage this carving out of a new relationship between the UK and the EU in 18 months let alone two years, there’s not a chance, there never was a chance.
"That’s not to say we can’t have symbolically left. But it means that we’ve got to get our heads round the idea that leaving may be a symbolic act and that lots and lots of the details will still remain to be sorted out, so the uncertainty will not have gone away."
Asked how long he thought it would take to strike a final deal, Lord O'Donnell said: "I can imagine it taking at least five years to get through all of the details. And I imagine some of the transitional arrangements may be longer than that."
However, Downing Street on Thursday insisted that Britain could finalise its withdrawal from the EU and agree a trade deal within two years.
A spokesman for the prime minister said: "We want there to be a resolution within the timeframe that we've set out... to exit the European Union with a deal that allows us the best possible access to trade with and operate in the single European market. That is what we are set on doing and we want to expedite that."