Unelected prime ministers would be forced to hold a general election within six months of taking office under proposals announced by David Cameron today.

The Conservative leader will promise to legislate to prevent a repeat of Gordon Brown’s three-year tenure at 10 Downing Street before he had to face the ballot box. On a campaign visit to Essex, Mr Cameron was also announcing a plan to provide political parties with taxpayers’ money to fund “postal primaries” in which voters will be able to choose election candidates.

The scheme would mean a £1.6 million cut in the annual budget of the Electoral Commission watchdog.

Gordon Brown continued Labour’s push for re-election with a public services rally in the East Midlands.

But like the Lib Dems’ Nick Clegg, the Tory leader was taking a break from electioneering later in the day to attend his younger sister Clare’s wedding to market research executive Jeremy Fawcus at a private ceremony in southern England.

It was Clare, aged 38, who introduced Mr Cameron to his future wife Samantha when they went on holiday together to Italy.

Three of the last five Prime Ministers – Mr Brown, John Major and James Callaghan – inherited power in the middle of a parliamentary term, rather than winning it in a general election.

Of those, only Mr Major has so far gone on to secure his own popular mandate at the ballot box when the Conservatives won the 1992 election.

Under Mr Cameron’s plans, a prime minister who takes office after the death, resignation or overthrow of his or her predecessor will be forced to face the electorate within six months.

Tories would amend the Septennial Act 1715, which requires elections to be held at least every five years, to ensure that Parliament is automatically dissolved six months after a change of prime minister for any other reason than a general election.

A new PM would be free to request a dissolution at any time during the six-month period, which would allow time for him or her to appoint a ministerial team and set out a programme for government.