LONDON — After one of the most passionately contested elections in decades, voters in Britain went to the polls on Thursday and appeared to have swung sharply, though not necessarily decisively, against the ruling Labor Party.
Early results from 650 House of Commons constituencies across the country pointed to a strong performance by the opposition Conservative Party, led by David Cameron, though the size of the increase in the Conservative vote raised doubts about the party's ability to win the majority needed to be sure of regaining power after 13 years of Labor control.
Cameron said late Thursday night that Labor had lost its mandate to govern, but this was formally unconfirmed.
What seemed sure was that the Conservatives would win the largest number of seats, probably dozens more than Labor, with the third party, the left-of-center Liberal Democrats, trailing in third. But without a majority, Cameron — and the country — could be heading for days of agonizing uncertainty as the two main parties set about trying to outmaneuver each other for power.
The first results announced showed a sharp swing from Labor to the Conservatives in seats in northern England that had been Labor strongholds for decades. Early returns from central and southern England suggested a similarly strong shift to the Conservatives — in one case of more than 9 percent.
Pollsters had said before the vote that a nationwide swing of 7 percent in the Conservatives' favor might be enough for a slim majority. The vote count showed a swing to the Conservatives that ranged from 5 to 11 percent.
So, late Thursday, it seemed possible that Britain was headed toward a hung Parliament for the first time since the 1970s, a situation in which no party has an overall majority and so cannot pass legislation without support from another party. It is unclear how this relatively unusual scenario will play out, but there are a number of possibilities.
First, the Conservatives, who by all pre-election polling were predicted to win the most seats, could try to form a coalition with one or more smaller parties. The leading contender for that role would be the Liberal Democrats, the third-largest party, who would relish the chance to play kingmaker but have been coy about how they would go about it.
Alternatively, the Conservatives could seek to make deals with individual members of all the small parties in Parliament — the Ulster Unionists, the Scottish National Party, Plaid Cymru and others. Some of those parties are traditional ideological opponents of the Conservatives, and it is unclear whether all the small-party votes combined would even be enough to allow the Conservatives to reach the 326 seats needed to form a majority.
It seems unlikely that Labor and the Liberal Democrats will win enough seats to form a majority coalition. But they could try to join to form a minority-party coalition government.
The leading wild card in all this is the Liberal Democratic Party. Buoyed by his unexpected surge in public opinion polls during the campaign, the Liberal Democratic leader, Nick Clegg, has intimated that he would be reluctant to enter into a formal deal with another party but that he would consider legislation on a case-by-case basis. He also has said the party that wins the most seats and the most votes is entitled to his party's "support," though he has not said what he means by that.
Many elements of Clegg's legislative program, which includes scrapping college tuition and overhauling the parliamentary election system, are anathema to the other parties, and it is unclear what kind of demands Clegg would feel he could make.
An interesting twist, British constitutional experts say, is that Gordon Brown, as the sitting prime minister, has the right to remain in office until another party can prove that it has the confidence of Parliament — that is, that it can amass enough votes to pass legislation.
For a final element of unpredictability, because the sitting prime minister has the right to call an election whenever he wants, it is impossible to plan for long-term cooperation on any sort of legislative agenda, an unhappy prospect for the junior partner in the coalition.