The UK would struggle to negotiate its exit from the European Union within the two-year time limit, according to a former cabinet secretary.
A Brexit win in June's referendum would lead to a "very complex process", Lord Gus O'Donnell (known to his mates as "God") has said.
And he described the prospect of winning support from all other EU nations for an extension to the two-year deadline to complete negotiations as "a bit scary".
"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," O'Donnell told BBC's Today programme.
"I'm in that camp that doesn't think we can do it in two years."
He added: "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 said that Greenland's 1985 decision to leave the European Community, which preceded the EU, offered the only precedent.
But, comparing Greenland's population to a crowd at Wembley, he said Greenland had one "issue" at stake: fish.
He said: "So with one issue, small population 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."