Britain will find it very difficult to negotiate its Brexit terms within two years
According to former Cabinet Secretary Lord O'Donnell, who served under three successive Prime Ministers between 2005 and 2011, negotiating an exit from the 28 nation bloc will be extremely hard.
"I'm in that camp that doesn't think we can do it in two years," said Lord O'Donnell to BBC Radio 4's Today on Tuesday.
"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."
According to the EU's Lisbon Treaty, a country gets two years to complete its withdrawal from the EU. This includes negotiating trade deals and other issues that will be affected from exiting the EU's Single Market.
Those supporting the "leave" campaign have consistently used the example of the positive effect withdrawing from the EU has had for Greenland.
However, Lord O'Donnell warned that you cannot compare Britain to Greenland due to the size of the nations and the different complexities in both economies.
"Greenland has a slightly smaller population than Croydon and it has one issue, and that's fish," said O'Donnell.
"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."