Boris Johnson’s worsening condition, and his admission to the Intensive Care Unit, come as a profound shock.

Across the nation there is a fervent and spontaneous outpouring of prayers and good wishes for his recovery, in which this site naturally joins.

Party political hostilities stand suspended as we watch the struggle of this most optimistic, energetic and generous-hearted figure.

It is too soon to start drawing any conclusions for the future from such an alarming and unexpected turn of events, but one may perhaps be permitted a glance at the past.

Johnson’s reluctance to relinquish the reins of power as he fell ill is in full accord with past practice, for hardly anyone with the drive to become Prime Minister possesses also the self-effacement needed to step aside on health grounds, except when this becomes absolutely unavoidable.

Theresa May did not step aside after grinding repeatedly to a halt with throat trouble during her party conference speech in 2017, but carried on for almost two more years.

Tony Blair did not regard his heart problems in 2004 as any reason to hand over to Gordon Brown, and only departed in 2006, like May overthrown by his own MPs.

Harold Macmillan did decide to stand down in 1963, unleashing a tremendous battle for the leadership, but soon discovered that although his political standing was already in sharp decline, his medical problems were not as severe as he had supposed, for he lived until 1986.

Winston Churchill had no intention, after suffering a severe stroke in 1953, of handing over to Anthony Eden.

As with Blair, the identity of the likely successor was an additional reason to keep going. Churchill doubted whether Eden – whom he had nominated as his successor in 1942 – would be up to the job, while Blair was anxious to stop Brown taking over.

Eden, who did at length become Prime Minister in 1955, stepped down on health grounds in January 1957, but the real problem was that his reputation had been destroyed in November 1956 by his mishandling of the Suez crisis.

His botched gall bladder operation had taken place in 1953, and left his health permanently impaired, but not to such an extent that he considered himself debarred from taking on the highest office, and one may note in his defence that he lived until 1977.

Andrew Bonar Law, who became Prime Minister after the overthrow by the Conservatives of David Lloyd George in October 1922, began the following year to have trouble with his throat.

In April, his voice grew so weak that he could not make himself heard in the Commons, and his doctor advised a Mediterranean cruise, which had to be broken off at Genoa, for Law was suffering from an acute pain in the side of his face.

His friend Lord Beaverbrook summoned the doctor to meet them in Paris, and here incurable throat cancer was diagnosed, though the word “cancer” was not mentioned in the patient’s presence. Law resigned in May 1923 and died in October.

In December 1905, when Sir Henry Campbell-Bannerman became Prime Minister, he was 69, and attempts were made by his Liberal colleagues to persuade him, for the sake of his health, to lead the Government from the House of Lords as a mere figurehead.

He rebuffed these intriguers, and proceeded in January 1906 to win a great election victory over the Conservatives. But his wife soon afterwards became very ill, and would only be nursed by him. Campbell-Bannerman stayed up night after night nursing her, which exhausted him, and in August 1906 she died in Marienbad.

From the summer of 1907, he suffered a series of increasingly serious heart attacks, and on 5th April 1908, by which time he was confined to bed in Downing Street, he composed a letter of resignation to Edward VII.

Campbell Bannerman died 17 days later, still in Downing Street, becoming the only PM or ex-PM to have perished there.

So a mortal illness compels a Prime Minister to step down, but anything less can be weathered as long as he or she continues to command a majority in the House of Commons.

It is encouragingly unusual for a Prime Minister to die in office. The most recent to do so was Lord Palmerston, who won an increased majority at the general election of July 1865, but was carried off by pneumonia in October of that year, two days before his 81st birthday, having caught a chill while out driving in an open carriage.

He was until that point a statesman of amazing vitality, who became Prime Minister at the age of 70, older than anyone else has entered Downing Street for the first time. Florence Nightingale said of him after his death: “He will be a great loss to us. Tho’ he made a joke when asked to do the right thing, he always did it.”

Daniel Finkelstein recently regretted in The Times that “we have never had a clear plan for replacing the prime minister in an emergency”.

But as Lord Lexden pointed out, in a letter of reply, any attempt to devise an emergency plan would itself cause grave difficulties.

One has only to glance across the Atlantic to see the problems which fixed terms, and a fixed line of succession, can bring.

In our parliamentary system, and with our more spontaneous idea of freedom, such matters cannot generally be settled in advance. And we in any case hope and trust we shall very soon see Johnson restored to his usual vigour.