May is apparently aiming to cash in on her 20-point lead over the main opposition Labour party to increase her majority and give her a stronger hand in the Brexit battles with Brussels ahead.
Britain started the formal process of leaving the EU last month, but negotiations are not due to begin for weeks -- giving the prime minister a narrow window for a lightning election campaign.
"We need a general election and we need one now," May said in a dramatic announcement outside her Downing Street office that caught the whole country off-guard.
"We have at this moment a one-off chance to get this done while the European Union agrees its negotiating position and before the detailed talks begin," she said.
Not another one!
The U-turn caps a tumultuous few years in British politics that has seen two historic referendums -- one on Brexit, one on Scottish independence -- and a prime ministerial resignation.
An early election would be the fourth big vote in four years and there are signs that the British public is beginning to suffer from election fatigue.
In a BBC video widely shared on social media, a woman called Brenda told her interviewer in Bristol when asked about the election: "Not another one! Oh for God's sake. I can't stand this!"
WATCH: UK opposition leader welcomes early election
MPs will vote on the move on Wednesday and May needs two-thirds of lawmakers to support her but Labour leader Jeremy Corbyn has already said he is behind the plan.
"We look forward to showing how Labour will stand up for the people of Britain," said Corbyn, a veteran socialist who became leader in 2015 and has strong support in the party's leftist grassroots but has struggled to secure wider appeal.
And recent opinion polls over Easter weekend showed the size of the challenge ahead of him.
The Conservatives polled at 38 percent to 46 percent, while Labour stood at 23 percent to 29 percent, according to the polls by YouGov, ComRes and Opinium.
Watch: Ben Lewis in London
May explained her policy U-turn with an attack on her domestic political opponents, many of whom support Britain's continued membership of Europe's single market, accusing them of "game-playing" over Brexit.
"I have concluded that the only way to guarantee certainty and stability for the years ahead is to hold this election and seek your support for the decisions I must take," May said.
Scottish First Minister Nicola Sturgeon, the leader of the pro-independence Scottish National Party which has 54 of 59 Scottish seats in the Commons, said May was making a "huge political miscalculation".
But in a boost to May, the IMF also on Tuesday significantly raised its forecast for Britain's growth in 2017 to 2.0 percent from a previous estimate of 1.5 percent.
The pound, which has fallen because of the Brexit vote and plunged ahead of May's announcement Tuesday, rose sharply against the euro and the dollar after it.
Stocks instead suffered their worst fall since the Brexit vote last June, since the rising value of the pound hits multinationals.
New battles loom
European Council President Donald Trump tweeted that he had had a "good phone call" with May following her announcement.
He then likened the latest twist in British politics to a movie by Alfred Hitchcock, often referred to as the "Master of Suspense".
"It was Hitchcock who directed Brexit: first an earthquake and then the tension rises".
The Brexit negotiations themselves are not expected to start until June at the earliest -- and Brussels said this timetable would not change.
The European Commission has said it wants the exit talks to be concluded by October 2018 at the latest, to allow time for the deal to be ratified.
WATCH: Scotland reacts to UK snap election
But while May secured the parliament's support to trigger Article 50 of the EU's Lisbon Treaty last month, eurosceptics and europhiles alike are gearing up for further battles over the details of the negotiations.
German Foreign Minister Sigmar Gabriel said in a statement that "hopefully" the new elections "will lead to more clarity and predictability in the negotiations with the European Union".
One European source said Tuesday that May would be better placed to enter the negotiations -- and give "concessions" -- if she had a stronger hand.
"We have some hope that this will lead to a strong leader in London that can negotiate with us with strong backing by the electorate," said another.
The other 27 EU leaders are set to hold a summit on April 29 where they will agree on a strategy for negotiating Britain's expected departure in 2019.