If Britain votes to leave the EU it would be “highly unlikely” exit terms could be negotiated within the required two years, according to the former cabinet secretary Lord O’Donnell.

“I’m in that camp that doesn’t think we can do it in two years,” O’Donnell told the BBC. He called the idea of requesting more time to complete the process “a bit scary”.

Gus O’Donnell was cabinet secretary, the most senior civil servant, for six years under Tony Blair, Gordon Brown and David Cameron.

O’Donnell has not officially declared himself in favour of remaining in the EU in June’s referendum, but his comments echo concerns from some remain campaigners who say the two-year timetable laid out under the Lisbon treaty could prove unrealistic. To be extended, all remaining EU members must agree.

“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,” O’Donnell said, indicating the process could linger for up to a decade.

He cited the example of Greenland, which voted in 1985 to leave the then-equivalent of the EU, the EEC.

“Greenland has a slightly smaller population than Croydon and it has one issue, and that’s fish,” O’Donnell said. “So with one issue … it took them not two years, but three. We have multiple issues. The idea that we can do it all in two years I think is highly unlikely.”

Those who support a UK exit from the EU argue that the process would be considerably more flexible, and that the UK could begin negotiations without formally triggering the deadline.