Despite the loss of at least 12 seats for the Conservatives, Mrs. May will try to form a working majority with the support of the Democratic Unionist Party, which won 10 seats on Thursday. With the 318 Conservative seats plus the D.U.P. seats, Mrs. May would have 328 votes — just above the 326 needed for a majority.
The D.U.P., a historically Protestant party that seeks to maintain Northern Ireland’s place in the United Kingdom, has close ties with the Conservatives, and it supported Britain’s withdrawal from the European Union. It was unclear what price the D.U.P. might exact for its support. “I certainly think that there will be contact made over the weekend,” said Arlene Foster, the party’s leader, “but I think it is too soon to talk about what we’re going to do.”
There is a precedent for this situation: The Ulster Unionist Party, another faction from Northern Ireland, helped shore up the government of John Major, a Conservative prime minister, from 1992 to 1997.
In some respects, the election on Thursday resembled the one in 2010, when the Conservatives won the most seats in a general election but did not have a majority of seats. They formed a coalition government with the Liberal Democrats.
In a coalition government, the junior partner takes ministerial seats, is part of day-to-day decisions by the cabinet and shares a platform with the governing party. In a minority government, in contrast, a smaller party agrees to support the governing party in votes on legislation, but it would not necessarily be part of the government.
Whatever emerges will most likely be more fragile than the coalition formed in 2010 by David Cameron, Mrs. May’s predecessor as prime minister, which lasted for five years.
And even if Mrs. May were to survive in the medium term, her authority has been badly damaged. She is certain to face demands from lawmakers in her own party that she change her leadership style and consult more widely. Nigel Evans, a senior Conservative lawmaker, blamed for the party’s so-called manifesto, or platform, over which Mrs. May had to reverse course within days, for the election failure.
Mr. Evans suggested that divisive proposals on financing of long-term care for the elderly — from which Mrs. May had to backtrack — would not have been included in the manifesto if Mrs. May had consulted more widely. “We didn’t shoot ourselves in the foot, we shot ourselves in the head,” he told the BBC.
Paradoxically, the Conservative Party actually increased its share of the total vote from 2015 — when it won a commanding majority in Parliament — but not by enough in key constituencies. Under Britain’s first-past-the-post system, what matters is not a party’s share of the overall vote, but simply who places first in any given constituency.
The share of the vote captured by Labour on Thursday — 40 percent — was significantly higher than what many parties that have formed governments in the past have won.
Analysts cautioned that this may reflect the volatility of British politics in the aftermath of the financial crisis, and that it might not point to a long-lasting revival in the fortunes of the two main parties.
In particular, the Conservatives and Labour have benefited from the collapse of votes for the right-wing, populist U.K. Independence Party, and the failure of the centrist Liberal Democrats to make a breakthrough.
“Two-party politics in the 1950s and 1960s was supported by an intellectual, cultural, infrastructure of class alignment and partisan alignment,” said Philip Cowley, professor of politics at Queen Mary, University of London. “These parties were class-based, people felt strongly affiliated to them, they voted for the same party year in year out – that is not going to be true this time.”
By the time of the next election, Mr. Cowley noted, votes could “churn again, back to another majority party or off to a minor party.”
The focus of British politics quickly shifted to Northern Ireland on Friday, where the D.U.P. is suddenly poised to act as kingmaker.
Brendan Halligan, chairman of the Institute of International and European Affairs in Dublin, said that a D.U.P. role in the British government could unsettle the intricate peace arrangements that make up the Good Friday Agreement of 1998, which largely ended the sectarian violence in Northern Ireland known as the Troubles. The government in Northern Ireland is currently divided because of a deadlock between the D.U.P. and its republican archrival, Sinn Fein.
“It will be a terrible dilemma for a new Tory government: The more they accommodate the D.U.P., the more they put out Sinn Fein, and the more they will come into disagreement with the Dublin government over north-south relationships,” Mr. Halligan predicted.
He said of the D.U.P.: “Their only real aim is to protect the union with Britain and prevent any drift towards a special status for Northern Ireland. They will put a stop to Sinn Fein’s attempts to push for that. That will draw a reaction from Sinn Fein, a knock-on effect in terms of forming a new power-sharing executive in Stormont.”
Andy Pollak, a former director of the Center for Cross Border Studies in Belfast, said he feared the consequences for Irish politics.
“The D.U.P. will always take the hardest of hard lines when it comes to Brexit or any slight compromise towards the republicans,” he said. “It’s all bad now. I don’t know what they will demand from the Tories for their support, other than a hard line on Brexit. The chances of a return to power sharing in the North are now lessened further. A lot of the D.U.P. were never happy with power sharing with Sinn Fein anyway.”