Negotiations for Britain to leave the European Union could last a whole decade, the former head of the civil service has said.
Gus O’Donnell argued it would be “highly unlikely” that a withdrawal would follow the timetable set out in EU rules.
“I'm in that camp that doesn't think we can do it in two years,” the former cabinet secretary told BBC Radio 4’s Today programme.
“We have to negotiate our entry to the single market, we have to negotiate our future relationship with the EU and then we have to negotiate our trade treaties with all other countries. So there's a lot to be done."
Asked how long it could take for the UK to put together such a deal, Lord O'Donnell cited a Cabinet Office document which warns of “a decade or more of uncertainty”.
Such a timescale would likely see at least two general elections pass during negotiations – and possibly a change of government, where elected parties might take different views on whether to leave the bloc.
The former cabinet secretary also argued that the French and German governments would be incentivised to make EU exit look more difficult because they were both facing anti-EU parties at their respective general elections.
France’s National Front advocates EU exit while Germany’s Alternative fur Deutschland only wants to pull only the European single currency.
Eurosceptic Conservative minister Dominic Raab told the same programme that a new deal could be produced “relatively soon”, however.
David Cameron, who is campaigning to stay in the EU, has previously described Brexit as a "leap in the dark".
Lord O'Donnell was previously the cabinet secretary between 2005 and 2011. He now sits as an independent crossbench peer in the House of Lords.
Britain is voting in a national referendum on whether to leave the EU on 23 June this year.
