The House of Commons voted by 522 to 13 to hold a general election on June 8 -- plunging Britain back into political uncertainty just weeks before the start of negotiations on leaving the European Union.
Addressing MPs beforehand, May said an early vote would strengthen her hand in dealing both with Brussels and domestic critics seeking to "frustrate the process" of Brexit.
"I will be asking the British people for a mandate to complete Brexit and to make a success of it," the Conservative leader said, to cheers from her lawmakers sitting behind her.
May stunned the country on Tuesday when she announced her plan for an early vote, despite having repeatedly said she would wait until the next election scheduled in 2020.
Riding high in the opinion polls, May is seeking to increase her slim majority of 17 in the 650-seat Commons before the battles begin with the EU over Britain's exit bill and future trade and immigration ties.
She says an early election would provide "certainty and stability" in the negotiations, which will now start after the vote.
Theresa May appeals to the parliament
In a taster of the campaign ahead, May traded barbs in the Commons with opposition Labour leader Jeremy Corbyn -- most likely for one of the last times before the election, after the prime minister ruled out attending any television debates.
Corbyn, whose deeply divided party is languishing behind the Conservatives in opinion polls, accused the government of "broken promises" on health, education and the economy during its seven years in office.
May hit back that Labour offered only "bankruptcy and chaos", but denied she was complacent, saying: "We will be out there fighting for every vote."
'Election landslide'
British elections are fixed in law but can be changed with a two-thirds majority in the Commons -- a threshold easily passed during Wednesday's vote.
Parliament will still sit for another fortnight, but party leaders wasted no time in hitting the campaign trail on Wednesday evening.
At a rally in south London, Corbyn pledged to fight for social justice in the Brexit talks and promised more investment in public services.
May meanwhile headed to the northern English town of Bolton to promise "the strong and stable leadership this country needs to take Britain through Brexit and beyond".
The prime minister, who took office after David Cameron resigned following the EU vote, is seeking a personal mandate for her plan to pull Britain out of Europe's single market.
In this image taken from video Britain's Prime Minister Theresa May speaks in the House of Commons in London. Wednesday April 19, 2017. Source: Press Association
Three weekend opinion polls put the Conservatives about 20 points ahead of Labour, while Britain second female prime minister also has strong personal support.
By contrast, Labour has struggled to form a strategy over Brexit, while Corbyn's left-wing leadership is opposed by many of his more centrist MPs.
Labour supported the start of the formal Brexit process last month, but has demanded certain conditions, such as retaining strong economic ties with the bloc.
That approach risks satisfying neither its traditional working-class supporters, many of whom backed leaving the EU, or its urban, pro-European members -- leaving many commentators predicting an electoral disaster.
Scotland's mandate
The election is the fourth major vote in four years, after last June's EU referendum, the 2015 general election, and the 2014 Scottish independence vote.
The smaller Liberal Democrats, who lost most of their seats in 2015 after entering coalition with the Conservatives, are hoping to capitalise on their strong support for the EU to win new support.
But one of last year's strongest pro-European voices, former Conservative finance minister George Osborne, will not seek re-election.
Meanwhile the Scottish National Party, which holds most of the seats in Scotland, is pushing its demands for a second referendum on independence in order to maintain close ties with the EU.
"Make no mistake, if the SNP wins this election in Scotland -- and the Tories don't -- then Theresa May's attempt to block our mandate to hold another referendum when the time is right, will crumble to dust," SNP leader and Scottish First Minister Nicola Sturgeon said.
The election announcement caused a rally in the pound, which had fallen since the Brexit vote, amid speculation that May will be returned with a stronger mandate.
Watch: May wants voters to go to the polls on June 8
