Former cabinet secretary Gus O'Donnell has angered Eurosceptic politicians by saying that Brexit is not inevitable.
Lord O’Donnell is no longer a member of the civil service, which must remain strictly neutral on matters of government policy, but his views are still highly influential.
He also said that whatever happens, the UK will have to keep some EU laws and rules.
Lord O’Donnell, now a crossbench peer, told The Times: "Lots of people will say, ‘We’ve had the referendum, we’ve decided to go out, so that’s it, it’s all over’.
"But it very much depends what happens to public opinion and whether the EU changes before then.
"It might be that a broader, more loosely aligned group, is something that the UK is happy being a member of."
But he also acknowledged that this was "not what people voted for".
Ukip MEP Douglas Carswell said Lord O’Donnell’s “contempt for democracy is shocking – and typical of the mandarinate in Whitehall.”
His words, he said, “Illustrate how our democracy has been subverted by career civil servants.”
Lord O’Donnell also said that implementing Brexit would be extremely complicated, and take years.
Leaving, he said, "means a huge administrative and legislative change because of all of those rules and laws and directives that have been implemented over this last 40 years".
He added: "My instinct is we will almost certainly stick with them (the EU rules and laws) and say, ‘OK, we’ll keep them for now’, so you can leave with everything in place."
