Cameron's Conservatives have won enough seats in the House of Commons to govern alone, ending their five-year coalition with the Liberal Democrats, but their majority is slim.
As a result Cameron risks being buffeted by rebellious lawmakers in his own party, particularly over a referendum on leaving Europe promised by 2017, as well as by the Scottish National Party (SNP), which has surged to become the third biggest party.
The 48-year-old prime minister's pledge to leave Downing Street before the next election in 2020 also leaves him vulnerable, experts say.
Cameron and his wife Samantha pose for pictures as they arrive back at 10 Downing Street in London on May 8, 2015, after visiting Queen Elizabeth II, a day after the British general election. The British PM's Conservative party on Friday won a majority in the House of Commons in the general election. - AFP Photo/Leon Neal
"Cameron returns now to 10 Downing Street for a maximum stay of three years," said Professor Patrick Dunleavy of the London School of Economics (LSE).
"He must step down by mid-2018 at the latest to allow his successor a decent run up to the polls.
"In the interim some bleak challenges will have to be faced by a government with a newly fragile majority."
EU referendum battle ahead
One big challenge is the in-out referendum on European Union membership which Cameron promised two years ago in the face of demands from eurosceptics in his centre-right party.
Christopher Howarth, senior policy analyst at think-tank Open Europe, said Cameron faced three tests -- renegotiating Britain's relationship with Europe, persuading his lawmakers to support the measures and selling them to the public.
He voiced optimism Cameron could persuade European partners who want to keep Britain in the EU of the need for reform but said the negotiation must be approached carefully.
"If he asks for too much, he might not get what he wants and look like a failure," Howarth said. "If he asks for too little, it may be written off by some."
While most Conservative lawmakers were likely to be persuaded to join Cameron in campaigning to stay in Europe, a hardcore of "awkward squad" eurosceptics would likely remain opposed.
"There will be MPs who aren't satisfied whatever the prime minister comes back with," Howarth said.
Opinion polls currently suggest that Britons will vote to stay part of Europe and Cameron will campaign to stay in, provided he can secure reforms such as tightening rules on immigration and state benefits.
But much could change during the referendum campaign in a country whose eurosceptic strain was highlighted at Thursday's election when the anti-EU UK Independence Party won some 12 percent of the vote.
SNP another headache
The spectacular rise of the pro-independence SNP is also likely to be a thistle in Cameron's side.
Nicola Sturgeon's party, which harbours a deep loathing of the Conservatives, won 56 out of 59 Scottish seats in the Commons, a ninefold increase on its previous tally of six.
SNP lawmakers including heavyweight former party leader Alex Salmond could cause trouble for Cameron at Westminster, particularly if the Conservatives' small majority shrinks further.
"When you get midway through a parliament, you get by-elections, you have got the Tory awkward squad (and) Europe which can divide the Conservatives," said Malcolm Harvey of Aberdeen University.
"If the Conservative party starts to fracture then you could see the SNP try and take advantage of that."
Harvey expected that the SNP would not use their position to push for another quick referendum on Scottish independence after the move was rejected in a vote last year, but would instead push for more powers for the devolved Scottish Parliament.
"This is a chance for them just to chip away at that so that when it comes eventually, the step to independence would be that bit smaller," Harvey said.
But he added that the SNP could "start to talk up an independence referendum again" if the rest of Britain voted to leave the EU against the will of broadly pro-European Scots.
If his majority is chipped away, Cameron could work with parties such as Northern Ireland's Democratic Unionists to keep his government in power.
John Major, the last Conservative premier before Cameron, used a similar tactic to cling to power after losing his majority in 1996, before being ousted in the 1997 election.
Although Cameron's previous coalition with the Liberal Democrats was dogged by squabbles, he may soon yearn for the relative stability it offered.
