In an unexpected statement at Downing Street, May said she was seeking a vote on June 8, less than halfway through the government's five-year term.
Opposition parties said they would not block the move, sending Westminster into full-throttle election mode.
The European Union brushed off May's announcement, saying it would not affect the negotiations on Britain's departure. But May's decision means that Europe's three most powerful nations -- France, Germany and Britain -- will be convulsed by internal election campaigns as the clock ticks on the two-year deadline to complete Brexit negotiations.
- MPs must approve decision to dissolve Parliament part-way through full term.
- Vote will be held in Parliament on Wednesday.
- May had full support of Cabinet and had spoken to the Queen.
- Opposition parties say they will not block move to hold election on June 8.
- Theresa May likely to substantially increase her slim majority.
May, who commands only a slim majority in parliament's lower House of Commons, said that a new mandate would strengthen her hand in Brexit talks.
A general election would end the attempts of opposition parties and members of the House of Lords to thwart her Brexit plans, she said. "If we do not hold a general election now, their political game playing will continue," she told reporters at Downing Street.
"At this moment of enormous national significance, there should be unity here in Westminster, but instead there is division. The country is coming together, but Westminster is not," she added. "We need a general election and we need one now."
Her decision is a sharp reversal of policy -- since taking over as Prime Minister, May had repeatedly ruled out an early election. May said she changed her mind on a recent walking holiday with her husband in Wales.
It is also a risky roll of the political dice. A fractious election campaign will reopen wounds barely healed after last year's EU referendum and give voice to those who oppose her strategy of pursuing a clean break from Europe.
May's Conservative Party currently holds 330 of the 650 seats in the House of Commons. She is expected to win an increased number -- opinion polls show support for the opposition Labour Party, led by Jeremy Corbyn, at record low levels.
Corbyn said he would not oppose the call for an election. "I welcome the Prime Minister's decision to give the British people the chance to vote for a government that will put the interests of the majority first," he said.
Under legislation introduced by the coalition government led her predecessor, David Cameron, an early election requires the support of two-thirds of MPs in the House of Commons. May said she would place a motion on Wednesday in the House of Commons calling for a vote on June 8.
A Downing Street spokesman said that May had the full backing of her Cabinet on calling the election and that the Prime Minister had spoken with Queen Elizabeth II on Monday.
May called on voters to throw their support behind her Conservative Party, adding that "every vote for the Conservatives will make me stronger" in Brexit talks.
Corbyn said he welcomed the decision to call for an election, even though his party is fractured over his leadership, widely regarded as lackluster.
Tim Farron, leader of the Liberal Democrats -- which was battered in the 2015 election and now holds just nine seats in the House of Commons -- said the election was Britain's chance to change direction.
The Liberal Democrats oppose Brexit, and Farron said he would push for as strong an association with Europe as possible -- a so-called "soft" Brexit -- including membership of the EU's free-trade zone, the single market.
The party said it had gained 1,000 new members in the hour after May's announcement. Labour also said it had gained 1,000 members Tuesday.
In Scotland, First Minister Nicola Sturgeon is likely to use the campaign to pursue her demand for a fresh independence referendum, after Scotland overwhelmingly voted to stay in the European Union.
"This announcement is one of the most extraordinary U-turns in recent political history, and it shows that Theresa May is once again putting the interests of her party ahead of those of the country," Sturgeon said.
Brexit talks loom
Britain voted in July last year to leave the union after 44 years of membership in a divisive and hotly contested referendum.
May officially began the Brexit process on March 29 by triggering Article 50, the legal mechanism needed to begin the divorce process and officially start talks with the EU.
The negotiations are expected to be tough and will likely take place over two years, though the more complex aspects of Britain's future relationship with the EU, such as trade, could take even longer.
May has struggled with not only the opposition, but with members within her own Conservative Party, who have been at loggerheads over what kind of Brexit the country should have.
The Prime Minister laid out her vision for Brexit in January and more formally later with a White Paper. But even that basic framework -- which spelled out that Britain would leave the EU's single market -- caused divisions in her party and involved several rounds of deliberations before a coherent plan could be presented.
The European Union appeared unwavered by the announcement. Preben Aaman, a spokesman for the European Council president Donald Tusk, said the EU would continue with its plans to adopt guidelines on April 29 for the Brexit talks.