Mrs May said she had "only recently and reluctantly" concluded that an election was desirable, and blamed opposition parties and the House of Lords for weakening her negotiating position with the EU.
Sources near to May told the "Daily Telegraph" newspaper she had already undergone a face-to-face TV interview about her decision for an early election.
European Council President Donald Tusk spoke to May and quoted British master of suspense Alfred Hitchcock's recipe for a good movie in likening her dramatic move to a thriller plot: "It was Hitchcock who directed Brexit: first an quake and the tension rises", the former Polish premier tweeted.
NEW YORK, April 18 Sterling jumped, along with gold, while stocks and the US dollar fell on Tuesday after Britain called a snap election for June, adding to investor concerns over geopolitical instability.
You have criticised the government's vision for Brexit, you have challenged our objectives, you have threatened to block the legislation we put before parliament.
May's Conservatives, who were split on the issue of European Union membership ahead of last year's referendum, are now far ahead of Labour, the main opposition party, according to opinion polls. Leaders of European Union states are due to adopt negotiating guidelines at an April 29 summit, and the bloc will prepare detailed plans for the talks with Britain by late May.
"But now 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".
"They are wrong", she said.More news: New Zealand tightens immigration rules in 'Kiwi-first' crackdown
"They underestimate our determination to get the job done and I am not prepared to let them endanger the security of millions of working people across the country".
The Fixed-term Parliaments Act was brought in to stop Prime Ministers calling a general election based on polls.
That is highly likely to happen on Wednesday.
With Labour perceived as weak on the issue of Brexit, Mr Corbyn will be keen to focus on anti-austerity policies such as these in an attempt to win over voters.
"Labour will be offering the country an effective alternative to a government that has failed to rebuild the economy, delivered falling living standards and damaging cuts to our schools and NHS", he said.
Polls give May's Conservatives a double-digit lead over Labour, which could have its worst election showing in decades. And as elections are so unpredictable, there is always the outside risk it could spark a reversal in the entire Brexit process. The Scottish National Party (SNP) could also retain its hold over the Scottish seats in Westminster, which could be seen as a validation of First Minister Nicola Sturgeon's call for a second Scottish Referendum.
The Scottish National Party now holds 54 of Scotland's 59 seats in the British Parliament, making it the third-largest party there.
The pound rose 0.1 percent against the US dollar after the announcement, to 1.257, recovering from a 0.4 percent drop an hour earlier.