The UK's former top civil servant Lord O'Donnell has warned it could take more than the proposed two years to negotiate a British exit from the EU – and said talks may even last a decade.
O'Donnell said it would be “highly unlikely” a withdrawal would follow the timetable set out in the rules governing the bloc.
The former cabinet secretary appeared to back a government document claiming an agreement could take 10 years, and said the prospect of negotiating extra time with EU nations to smooth the process was “a bit scary”.
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The Lisbon Treaty states that countries opting to leave would have two years to hammer out the terms of an exit agreement.
An extension would only be granted if every member state agreed – otherwise the arrangement would automatically switch to so-called “World Trade Organisation” rules imposing tariffs on UK exports.
Lord O'Donnell said: "I'm in that camp that doesn't think we can do it in two years."
He added: "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."
Speaking to Radio 4's Today programme, he said the Lisbon Treaty was "a rather biased playing field" tilted in favour of the EU.
"Obviously at the end of two years anything we haven't negotiated has to be extended by unanimity of a vote excluding us, so that's a bit scary,” he added.
Citing the case of Greenland, which took three years to negotiate its exit from the European Community despite its small population, Lord O'Donnell said: "Greenland has a slightly smaller population than Croydon and it has one issue, and that's fish... We have multiple issues. The idea that we can do it all in two years I think is highly unlikely."
And he argued that EU leaders fighting elections would be incentivised to make a British exit from the bloc more difficult.
"Let's just imagine the guys on the other side of the table – countries like France and Germany. Both of them next year will be fighting elections," he said.
"They have anti-EU parties that they will be fighting against. Do you think the politics will be such that these countries, these leaders, will want us to be seen to be having great successes from leaving?
"I'm afraid the politics works completely the wrong way for us."
Asked how long it could take for Britain to forge a deal if it votes to leave on 23 June, Lord O'Donnell referred to a Cabinet Office document which warned of “a decade or more of uncertainty”.
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Anti-EU group Vote Leave has argued the UK could negotiate its exit from the bloc at any time without triggering the formal process.
Eurosceptic minister Dominic Raab on Wednesday argued the political will and mutual interest in the EU to reach an agreement with the UK was "very strong".
"Of course we would strike a new deal and relatively soon with transitional arrangements if necessary," he told the Today programme.
And he said Greenland's ability to reach an agreement despite its population size suggested Britain "would be taken far more seriously".
Lord O'Donnell's intervention came after the Institute for Government think tank, which focuses on the effectiveness of government and the civil service, warned that the EU vote risked "distracting" ministers from the day-to-day running of their departments.
The think tank's deputy director Julian McCrae – who has been sharply critical of the recent Single Departmental Plans, which were designed to match manifesto commitments with plans for departmental reform – said that ministers needed to show leadership in spite of major events.
"With the EU referendum and the fall-out from Iain Duncan Smith's resignation, there is a tendency to try and put forward new initiatives," he told the BBC.
"The real danger is that ministers themselves become distracted. The civil service responds to what ministers are focused on. It is quite important that people are checking if they have got real plans in place, are they working, is it going right. Look at something like Universal Credit in the last parliament, that ran on for about two years, not going well, not really on track.
"It is quite important that both sides work together in this and understanding where do you need ministers to actually get things done in government. Setting out the priorities, what you have to really focus on, is something really only ministers can do."