Unelected prime ministers would be forced to hold a general election within six months of taking office, under proposals being 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.
Speaking on a campaign visit to Essex, the Tory leader will also announce a plan to provide political parties with taxpayers' money to fund "postal primaries" in which voters will be able to choose their candidates for general elections.
The scheme would mean a £1.6 million cut in the annual budget of democratic watchdog the Electoral Commission.
Mr Cameron's announcements come as Mr Brown continues Labour's push for re-election with a public services rally in the East Midlands, while Liberal Democrat leader Nick Clegg takes a day off the campaign trail to be with his three sons, who returned to the UK last night after being stranded in Spain due to the volcanic ash cloud.
The Tory leader will also take 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, 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, by winning the election of 1992.
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 of taking over.
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 and for Parliament to deal with any outstanding business, said Tories. The legislation would be flexible enough to avoid triggering elections at inconvenient times like Christmas or the summer holidays.
That flexibility would have been needed if the legislation had been in place when Tony Blair quit in 2007, as Mr Brown might otherwise have been forced to dissolve Parliament on December 27.
The Prime Minister is widely believed to have abandoned plans for an election in the autumn of 2007 because of a sharp spike in support for the Tories after they unveiled plans to take estates under £1 million out of inheritance tax.
Mr Cameron will also unveil plans to "open up democracy" by funding 200 all-postal primaries across the country over the course of the next Parliament at a cost of about £8 million.
The Tories were the first party in British political history to select one of their candidates by an all-postal ballot of a constituency when Sarah Wollaston was chosen in Totnes, Devon, in July last year. A second candidate, Caroline Dinenage, was later selected by the same method in Gosport.
Under the plans being outlined by Mr Cameron today, postal primaries would be funded by the state at an estimated cost of £40,000 each. The cash would be shared out between parties which take up seats in Parliament in proportion to their share of the vote in the general election.
Parties would select the constituencies where they want to hold primaries and put forward a shortlist of four or more candidates for a public vote. Everyone on the electoral roll in the constituency, regardless of political allegiance, will receive a ballot paper and a freepost envelope to return it in.
The Tory scheme envisages about £1.6 million a year being spent on the primaries, funded by cuts in the £23.5 million Electoral Commission budget.