"The Prime Minister called the election because she wanted a mandate and the mandate is that she has lost seats", he said in his victory speech, claiming on Twitter earlier that the Labour party had "changed the face of British politics".
May had sought to capitalize on Britons' desire to quit the European Union and the perceived weakness of opposition Labour Party Leader Jeremy Corbyn. But in one of the most sensational nights in British electoral history, a resurgent Labour Party denied her an outright win, throwing the country into political turmoil.
The results of the UK's general election are in. but there wasn't a clear victor. Perhaps we mean that the United Kingdom will be forced to accept membership on the European single market? Instead, the results have thrown up a hung parliament with May's Conservative Party losing its majority. She said Brexit talks would begin on June 19 as scheduled, the same day as the formal reopening of parliament. She alienated core Conservative electors and the party's manifesto was poorly received.
She told BBC Radio Wales: "We're still saying quite clearly that we would be ready to take over if Theresa May cannot cobble something together, and we're very doubtful that she can".
While May has officially announced plans to form a government, it does not yet mean she will be our next Prime Minister. It said Britain was "effectively leaderless" and the "country all but ungovernable". She repeatedly touted her "strong and stable leadership" throughout the campaign, but the voters weren't buying her as a strong and stable figure.
"But you've also got to convince them of your credibility that you can actually move from protesting about a government into being the government". May said outside Downing Street after returning from Buckingham Palace, where she received the Queen's permission to form a government, though the Conservatives lost at least 12 seats.
She said talks with Britain on its withdrawal from what will now be the 27-member European Union would start "in the coming days". If she is to succeed in delivering the wishes of 52% of the public and take Britain out of the European Union, she must find a way to secure the full support of her party to pass legislation preparing for and enacting the departure.
The Conservative party will form a minority government with the support of the Democratic Unionist Party (DUP) of Northern Ireland, having failed to secure the 326 MPs required for an overall majority in the House of Commons.
The two parties are broadly politically aligned, but it remains to be seen what price the DUP will seek to extract for its support.
After winning his own seat in north London, Mr Corbyn said Mrs May's attempt to win a bigger mandate had backfired.
Ruth Davidson, the Conservative leader in Scotland, said she had asked May for assurances that there would be no attack on gay rights after a deal with the DUP.
It was not immediately clear what the DUP's demands might be and one DUP lawmaker suggested support might come vote by vote. Despite many calls for her to resign and let a new leader step forward, May has said she will continue to lead because "what the country needs more than ever is certainty".