Cameron's Conservatives are the clear favourites to win more seats than any other party in the May 7 vote, but betting firms predict that will not be enough to keep him in office.
Indeed, bookmakers reckon a minority government of the current main opposition Labour Party, led by Ed Miliband, is the most likely outcome.
Betting chain William Hill now have a hung parliament at odds of 1/8, meaning they reckon there's an eight in nine chance that it will happen.
However, when it comes to forming a government, they reckon a Labour minority administration -- backed up by smaller, left-wing parties -- is the most likely election outcome.
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If no single party wins a majority, it is not necessarily the party with the most seats which gets to form the next government.
Labour could win fewer seats than the Tories and still take power if it can secure the backing of other, smaller parties to boost its numbers and create a parliamentary majority.
"The post-election government is where it gets very, very messy."
Paddy Power have a Labour minority at 13/8, then a Conservative minority at 5/2 and a Conservative majority at 6/1, the same as a second Conservative-Liberal Democrat coalition. A Labour-Liberal coalition is 8/1, then a Labour majority 16/1.
A Conservative-Labour coalition is 25/1, very much an outside chance.
Bridge said: "Labour are ever so slightly favourites for a minority government because when you look at who the Tories and Labour can buddy up with... Labour can obviously turn to the SNP (Scottish National Party) very easily."
"The good news for Mr Miliband is that for the first time Hills have him as 10/11 joint-favourite, along with David Cameron, in their market on who will become prime minister," said William Hill spokesman Graeme Sharpe.
"The bad is that Hills offer 4/7 that he will no longer be Labour leader on January 1, 2016 -- 5/4 that he is.