May is now facing questions over her judgement in calling the election three years early and risking her party's slim but stable majority of 17.
May will visit Buckingham Palace at 12:30 pm (1130 GMT) as leader of the Conservative party, which won the most seats in Thursday's general election but lost its parliamentary majority.
Guenther Oettinger, a German member of the European Commission, said Friday that the election result had left it unclear whether negotiations between the United Kingdom and Europe on Britain's exit from the EU could be launched on June 19 as planned.
With 649 of 650 seats in the House of Commons declared, the Conservatives had 318 to the Labour Party's 261.
With just a handful of seats left to declare, Thursday's poll shows gains for the opposition Labour Party.
Nevertheless there were reports Friday that the Conservatives had struck a coalition deal with the Democratic Unionist Party, a conservative party in Northern Ireland whose 10 MPs would give a Conservative-led coalition the majority required to govern. For May, who went into the campaign expecting to win a landslide, even a narrow win later in the night would leave her badly damaged. Her main opponent - Labour Party leader Jeremy Corbyn, known for his left-wing views - was stumbling from mishap to mishap, unable even to muster solid support from his party's own lawmakers.
"I would have thought that's enough to go, and make way for a government that will be truly representative of all of the people of this country", he added. Her position is vastly weakened, and not only with her own party.
Anna Soubry, a Conservative MP, said May would have to consider her position.
A veteran of the Liberal Democrats party, Vince Cable lost his Twickenham seat two years ago, to Conservative Tania Mathias.
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Shadow foreign secretary Emily Thornberry said Labour "could form the next government" and would attempt to do so as a minority government if results allowed, rather than seeking to form a coalition with other progressive parties like the Lib Dems. If the political stalemate continues, there is the very real possibility of another general election.
As a former Labour activist, looking at last night's dramatic election night in Britain through a "tribal" Labour lens, it was a thoroughly splendid night because any bad election result for the Tories is a cause for celebration.
The Conservatives' campaign slogan - promising "strong and stable" leadership - rarely rang true during May's rocky campaign. By contrast, Labour performed far better than expected. Thus, the Tories may have a lead of 5.2 percent intent voters.
Ms May and Mr Corbyn both cast their votes in their respective constituencies of Maidenhead, southern England, and Islington, north London.
It is believed the Prime Minister will ask the sovereign for permission to form a government in the wake of the election.
Not necessarily. A majority coalition is a formal agreement between two or more parties who between them have more than 323 MPs.
Although Brexit was supposed to be front-and-center during the campaign - the Conservatives advocating for a so-called "hard Brexit", with Britain severing almost all ties with the European Union, and Labour for a "soft" one - the subject scarcely featured in debate.
PM Theresa May was fighting to hold on to her job on Friday as British voters dealt her a punishing blow, denying her the stronger mandate she had sought to conduct Brexit talks and instead weakening her party's grip on power.
A major force in the 2015 elections and a driver of last year's Brexit referendum result, UKIP was weakened by the departure of its media-savvy leader Nigel Farage, by internal rivalries and by May's pitch to its voters on Brexit negotiations.
Samuel Tombs, analyst at Pantheon Macroeconomics, says the pound could drop further, to $1.2600, where it was trading before May announced the election in April. The formal Brexit negotiations were scheduled to start on June 19, when UK Brexit secretary David Davis would meet with European Union chief negotiator Michel Barnier.