Not only was he Prime Minister “at the time of the most significant health crisis in Britain for 100 years” but, by forcing through a hard Brexit and transforming the Conservative Party, he “will have shaped British history, its economy and foreign policy, for the entire middle 21st century”, said Sir Anthony Seldon, author of The Impossible Office? and a biographer of multiple recent premiers.

Of course, Mr Johnson may yet cling on to the keys of Downing Street for months, perhaps even years. The real question is whether he has passed the tipping point from which prime ministers do not recover, no matter how long they stay in office.

Rather than Sir Anthony Eden, Dr Garnett offers up Sir John Major as the best comparison. While a steady flow of sleaze allegations damage Sir John’s electoral hopes, the real blow to his popular support came just five months after his 1992 election victory, when Britain crashed out of the Exchange Rate Mechanism on Black Wednesday.

“He went from hero to zero overnight,” points out Dr Garnett, with public confidence in the Conservatives never recovering. The difference then was that, with years to go until the next election, Tory MPs were not thinking: “My God, I’m going to lose my seat.” Today’s Conservative MPs do not have that luxury.

As with all historical parallels, this one is far from perfect. For Sir John, his whole party was tarnished by Black Wednesday, but leadership hopefuls such as Rishi Sunak might hope to escape any flack for Number 10’s partying.

Moreover, as Dr Garnett points out, Black Wednesday, like Suez, was a single catastrophic event. Mr Johnson has been undermined by a series of scandals.