THE defence of Boris Johnson by some of his Tory MPs has come down to the ridiculous assertion that there should not be a change of leader in a time of crisis.

As anyone with even a slight grasp of British political history knows, such a craven and frankly ludicrous "defence" flies in the face of the facts of previous party leadership changes that swept numerous occupants of No 10 out on their ear in Downing Street.

Telly historian Dan Snow – no fan of Johnson - tweeted his take on it last night: “Churchill replaced Chamberlain the day Hitler invaded [western] Europe.”

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He then goes back to the 18th century tracking other changes of prime minister at times of crisis, including David Lloyd George replacing Herbert Asquith in December 1916, three months after the horrendous casualties at the Somme saw Asquith’s ability as a war leader seriously questioned.

His fellow Liberal David Lloyd George had threatened to resign from the Cabinet if Asquith didn’t go, and he ganged up with the Conservative leadership to replace Asquith.

It was the same problem for Neville Chamberlain in 1940. The great appeaser never looked remotely comfortable as a wartime leader, and with his majority in the House of Commons reduced in a vote of confidence called by Labour after the debacle of the Norway campaign, Chamberlain decided himself that he should go and advised the King to let Churchill into No 10. Churchill then formed the National Government of all parties, which he led to victory over the next five years.

The National: Tory PM Neville Chamberlain was ousted during the Second World War and replaced by Winston ChurchillTory PM Neville Chamberlain was ousted during the Second World War and replaced by Winston Churchill

Incidentally, Johnson appeared not to know about Leopold Amery’s famous speech to Chamberlain on May 7, 1940. Let me quote it directly: “This is what Cromwell said to the Long Parliament when he thought it was no longer fit to conduct the affairs of the nation: You have sat too long here for any good you have been doing. Depart, I say, and let us have done with you. In the name of God, go.”

When David Davis quoted Amery in the Commons a week ago, Johnson replied: “I must say to him, I don’t know what he is talking about. What I can tell him, I don’t know what quotation he is alluding to.”

As a biographer of Churchill, for Johnson to say he didn’t know the quotation is either another example of his mendacity or complete incompetence – it was one of the most famous things ever said in Parliament.

You don’t have to go back to the Second World War for examples of the Tories changing their leader and Prime Minister during or after a crisis.

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The most famous coup against a sitting Tory PM was the ousting of Margaret Thatcher which most certainly did take place at a time of crisis. On August 2, 1990, Saddam Hussein sent his troops – the fourth largest army in the world - to invade and occupy Kuwait. Thatcher and US president George Bush were the first world leaders to react, sending troops into Saudi Arabia in what became Operation Desert Shield.

The world knew how high the stakes were as Saddam "appropriated" Kuwait so that he could get his hands on that country’s oil riches. Yet even as the UK and its allies prepared for war, the Conservatives were imploding over the issue of Europe. The preparations for the inevitable war in the Middle East made not a jot of difference to the men who wanted change at No 10.

The National: Margaret Thatcher stepped down as prime minister due to turmoil within her own party at a time when the UK was sending troops into the Middle East Margaret Thatcher stepped down as prime minister due to turmoil within her own party at a time when the UK was sending troops into the Middle East

Having brought riots on the streets with her Poll Tax, Thatcher’s hawkish stance against the European Exchange Rate Mechanism and more European integration brought the crisis to a head. She effectively sacked then foreign secretary Geoffrey Howe who memorably savaged her in his resignation speech on November 13, 1990. That speech directly influenced Michael Heseltine to challenge for the leadership, which he did the following day.

The Conservative MPs voted on Tuesday, November 20, and the ballot revealed the level of her unpopularity – just 204 out of 372 voters backed her, a mere 54.8%. The Thatcher cause was holed below the waterline, and she decided to go rather than be voted out. John Major became party leader on November 27 and prime minister the following day, and he made the Gulf War his priority which stood him in good stead when he won the 1992 General Election.

Another Tory PM ousted by his own side was Sir Anthony Eden. Officially he resigned on January 9, 1957, because of ill health but he was already under crushing pressure from his own ministers to go because of the disastrous UK involvement in the Suez crisis in late 1956. Interestingly it emerged later that he had misled Parliament about the extent of the UK’s collusion with Israel and France over the invasion of Suez.

A Tory Prime Minister suspected of misleading Parliament who quits rather than be humiliated? Could history be about to repeat itself?