When should an 'unelected' PM go to the polls?

But during his speech at Palmer's College in Thurrock seven years ago, he made one very interesting – and long forgotten – pledge. He said that, had he won a majority back in 2010, any future Prime Minister who took office mid-term would be required to call a general election within six months.

The policy was, of course, a dig at his predecessor Gordon Brown, who had taken over from Tony Blair in 2007, just two years after his predecessor had promised to serve "a full second term".

Of course, Mr Cameron failed to gain a majority in 2010, and the promise was quietly forgotten, but it raises the interesting question of how long an "unelected" prime minister can remain in office before they outstay their welcome.

About 18 months, recent history would suggest. Of course, students of our constitution would point out that no prime minister is elected by the people. Rather, the public elects constituency MPs, and it falls to them to elect a prime minister. However, there is no doubt that party leaders play a key role in how people vote in general elections, and that a prime minister who takes over mid-term without having led their party into a general election, will always face awkward questions about their legitimacy. This may well have been a major factor in Theresa May's shock decision to go to the country three years early.

Mrs May, who is known for analysing the facts carefully before coming to a decision, will no doubt have been mindful of how Mr Brown's brief honeymoon period rapidly went sour after he decided against going to the polls.

When Mr Brown replaced Mr Blair 10 years ago, it seemed like he could do no wrong. By this time Mr Blair was a deeply unpopular figure, largely due to his handling of the Iraq war, and his party had been trailing the Conservatives – revitalised by the youthful and energetic Mr Cameron – for several months. But after Mr Brown took over, the polls were instantly reversed, with Labour at one point enjoying an 11-point lead, and speculation was rife that Mr Brown was about to go to the country. However a successful Conservative conference, followed by a poll which gave the Tories a narrow lead, persuaded Mr Brown to abandon his plans ­– a decision he would live to regret. Never again would he be in such a strong position to seek a mandate of his own.

Mr Brown's predecessor of nearly 30 years before could no doubt have told him a similar story. In September, 1978, it was widely predicted that Jim Callaghan would use his address before the TUC conference to announce an autumn general election. But, in what with hindsight seems like a terrible act of hubris, he instead performed a rendition of the music hall song Waiting at the Church, a taunt aimed at Margaret Thatcher's Conservatives, reminding them that he alone would decide the date of the next election. Months later Britain was rocked by the Winter of Discontent, which saw the country gripped by industrial unrest, and Callaghan was force to call an election the following May when his minority government lost a vote of confidence in the Commons. His act of bravado resulted in 18 years of Conservative government.

In between Callaghan and Brown was John Major, who took over from Margaret Thatcher following her resignation in November 1990. Within weeks of taking office he found himself leading the country into war, following the Iraq's invasion of Kuwait, and for a brief time his popularity soared. Yet, despite clear leads in the opinion polls for much of 1991, he declined to call an election that year, and many thought he had left it too late when he eventually went to the country in April 1992. By this time, the polls were extremely close, with many showing a narrow lead for Neil Kinnock's Labour Party, and it looked like Major might have missed the boat. However, he surprised many with his high-energy election campaign – which saw him take to the streets with a soap-box and a loud hailer – and pulled off a victory against the odds. The next five years would not be quite so easy though...

Going back further into history, both Alec Douglas-Home and Harold Macmillan were appointed mid-term having not gone into an election as leader

Macmillan took over from Anthony Eden in 1957 following the Suez Crisis, and presided over an economic boom which saw him win the 1959 General Election by a landslide. However, he caused something of a constitutional crisis when he resigned from ill health in 1963. At that time, the Conservative Party had no formal mechanism for electing a leader, and Macmillan was forced to consult the Queen for advice. Eventually, Lord Home agreed to renounce his peerage and stand for parliament, enabling him to become prime minister as just Sir Alec Douglas-Home. It was not a happy tenure; forced to call a general election the following year, his tenure lasted just 363 days, defeated by Labour's dynamic young leader Harold Wilson.

There were four "unelected" prime ministers during the first half of the 20th century, who enjoyed varying degrees of success. David Lloyd George was appointed in 1917, during the latter stages of the First World War, after his predecessor Herbert Asquith came under mounting over his handling of the war effort. He went on to win the 1918 General Election by a landslide.

Winston Churchill also became prime minister during wartime, replacing Neville Chamberlain as both premier and Conservative leader as part of a deal with Labour and the Liberals to form a unified coalition government. But while he was hailed as a great war leader, he failed to turn this popularity into domestic votes. He was defeated at the hands of Clement Attlee's Labour Party in 1945. Chamberlain had himself only been appointed prime minister some two years earlier, following Stanley Baldwin's resignation in the wake of the abdication crisis.

Indeed Bewdley-born Baldwin's first term as prime minister might serve as a timely reminder to Mrs May that calling an early general election is not without its risks.

Baldwin was appointed prime minister by King George V in May 1923, following the death of Andrew Bonar Law. He called an election the following December so he could get a mandate to introduce protectionist tariffs, but the result was inconclusive, leaving his Conservatives with 258 seats, Labour 191 and the Liberals 159. The following month he was defeated in a motion of no confidence and resigned.

He would later enjoy two further stints as prime minister, but his short-lived first ministry shows that calling a snap election to gain support for a controversial new policy is no guarantee of success.