David Cameron's former media chief, Sir Craig Oliver, has defended his ex-boss's decision to call the EU referendum, calling it "inevitable".
He said the ex-prime minister had to hold the vote to overcome threats from UKIP and rebellious Conservative MPs.
The 23 June vote went against Mr Cameron and he resigned the next day.
His apparent calm afterwards showed his "instinct was to be the leader" and not that he was "chillaxed about it", Sir Craig told an audience in London.
The promise of a referendum was part of the Conservatives' 2015 general election manifesto, meaning that when the party won a majority it was duty-bound to hold it.
Sir Craig, who worked for Mr Cameron from 2011 until this year, said: "If David Cameron had ignored the calls for a referendum, a great boulder would have been placed in the road of British politics."
He told the audience at the London School of Economics: "A referendum in this country was inevitable."
The Remain campaign, which Mr Cameron backed, lost by 51.9% to 48.1%, leading critics to question why the referendum was promised in the first place.
Asked if the former prime minister had been "too relaxed" following the vote, Sir Craig said: "His natural instinct was to be the leader - in charge. That's not being chillaxed about it."
He was also critical of Labour leader Jeremy Corbyn's role in the the Remain campaign, saying he had not helped Mr Cameron "get over the line", as he had assumed the side would win and he did not want to help the Conservatives.
Mr Cameron stood down as an MP in September.