BY the nature of the Prime Ministership and the vagaries of the unwritten British constitution, a man or woman can have a long stay in No 10, or sometimes they are hardly in the door before they have to send for the removal vans.

So will Boris Johnson get his feet under the table and enjoy that fabled bed he’s trying to obtain, or will he be out on his ear very shortly?

There’s been much speculation that it will be the latter, but how long will Johnson have to reside at No 10 Downing Street before he can say he outlived the incumbent with the shortest residency?

Outlived is the operative word, because the shortest prime ministerial reign was that of George Canning, whose life was tragically cut short after he had served only 119 days in 1827.

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Canning might never have got the job had Lord Castlereagh not merely wounded him during their infamous duel in 1809. Both were Tory members of the Cabinet, and it is thought that Castlereagh, a crack shot, deliberately aimed at Canning’s leg rather than kill him.

Castlereagh committed suicide in 1822, fearing the exposure of his homosexuality, by which time Canning was proving to be something of a political genius. So much so that he was made Prime Minister ahead of the Duke of Wellington and Sir Robert Peel by King George IV.

Plagued by ill health, he took office on April 12, 1827, and died of pneumonia less than four months later on August 8. In order for Canning’s record to survive, Johnson will need to survive in No 10 until November 20.

Others who were Prime Minister for short periods included Canning’s successor, Frederick John Robinson, Viscount Goderich, who could not hold together the coalition which Canning had minted.

He resigned after just 130 days in office on January 8, 1828, and thus holds the record for the shortest time as Prime Minister by a Premier who did not die in office.

Though born in Canada, Andrew Bonar Law is considered Scottish because his emigrant family sent him back here in 1870 to live in Helensburgh with his aunt. Always known as Bonar, he attended the High School of Glasgow but went into business rather than head for university.

He became an MP in 1900 in Glasgow Blackfriars, and after the First World War he represented Glasgow Central.When the Tories ended their coalition with Lloyd George, Bonar Law became leader and PM on October 23, 1922. He lasted 211 days before resigning on May 20, 1923, because of the throat cancer that killed him later that year.

The National:

After the Second World War, most Prime Ministers have lasted at least a year, though Anthony Eden, Gordon Brown and Theresa May (above) did not get long service awards.

Sir Alec Douglas-Home is the only post-war Premier to have lasted less than one year in office. The Old Etonian graduate of Oxford University – just like Boris Johnson – was selected by the ‘men in grey suits’ to succeed Harold MacMillan, but he promptly lost the 1964 general election to Harold Wilson and Labour after 363 days in office.

The grey suits no longer exist, do they? Instead 159,000 Conservatives got the chance to elect Boris Johnson. It remains to be seen how long he will last in the top job he’s always wanted.