Millions of people were still to be convinced about swapping the existing first-past-the-post system for the alternative vote (AV), the Prime Minister conceded.
But he said a Yes vote for the AV would usher in a system that would be "undemocratic, hopelessly unclear and much more complicated than what we have".
Speaking to an audience in Maidstone, Kent, ahead of the AV referendum on May 5, Mr Cameron said there were "obvious arguments" for sticking with the current system.
He said: "It is a system that is so clear that it can be summed up in one sentence - we all vote for our favourite and the one with the most votes wins. Simple.
"It is a system so democratic that each person's vote has the same value, has the same worth. You only get to vote once.
"That's not the case with AV, and it's a system so decisive that time and again it has allowed the British people to do something which is vital in a democracy.
"And it may be odd for a Prime Minister to say this but it is true. We have got to be able to kick out tired governments. Whether in 1979 or 2010, that is exactly what has happened."
He went on: "Don't swap the current system that is clear, simple and decisive for something that can keep dead governments on life support machines."
He spoke after the Cabinet met today for the first time since tensions between the Tories and Lib Dems over the AV referendum reached boiling point.
Downing Street said it was "business as usual" around the Cabinet table this morning, despite the exchange of fire between Lib Dem and Tory ministers over the weekend.
Deputy Prime Minister Nick Clegg used an interview over the weekend to launch his most direct attack yet on Mr Cameron, describing his defence of first-past-the-post as the "death rattle of a right-wing elite, a right-wing clique who want to keep things the way they are".
Tory backbenchers stoked up the row last night by branding Mr Clegg and his Lib Dem Cabinet colleague Chris Huhne - who has raised the prospect of legal action over supposed "untruths" from the No campaign - "whingers" and "bad losers".
But former foreign secretary Margaret Beckett, who is president of No to AV, said: "With just days to go, it is more important than ever that we put aside party differences for the good of the country and that everyone comes out to vote against the unfair and expensive AV system on May 5.
"I strongly urge people to look at the two voting systems in this contest and they'll see that our current system is simple, fair and decisive, whereas AV is an expensive and complicated political fudge."
Mr Cameron also urged voters to enjoy the royal wedding but reminded them of the importance of the upcoming elections.
"By all means enjoy the royal wedding, have that street party and all that will take place on Friday but these are really important elections," he said.
Earlier, Downing Street said there was no discussion of electoral reform during the Cabinet meeting, which focused on the economy, the upcoming royal wedding, Afghanistan and the situation in the Middle East and North Africa.
Asked about reports that Liberal Democrat ministers had complained they were being sidelined within their departments by Conservative colleagues, a spokeswoman said: "The coalition is working very well. I don't recognise these remarks."
The Times today reported that an agreement promising Lib Dem backbenchers access to papers and early sight of statements was being blocked in some departments.
It said in one large spending department, a Lib Dem minister was being deliberately frozen out of discussions by his Tory secretary of state.
The paper quoted an unnamed Tory as saying that the junior coalition partners were like "yapping dogs" which had to be tolerated but could be largely ignored.
Lord Mandelson later urged Labour supporters to "think strategically" when casting their votes in the referendum.
"If David Cameron, having opened the way to this referendum through his negotiations with the Liberal Democrats, were to see a Yes vote his party would never forgive him," he told Sky News' Boulton & Co.
"He has many critics, many detractors in the Conservative Party, they are desperately searching for some weapon to use against him and to pitch him out.
"If there was a Yes vote that weapon would be placed into the hands of his critics and I think they would use it."
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