In an unexpected statement at Downing Street, May said she was starting the process of calling a vote, less than halfway through the government's term, BBC reported.
Britain needed certainty, stability and strong leadership following the EU referendum, she said.
Explaining the decision, May said: "The country is coming together but Westminster is not."
There will be a Commons vote on the proposed election on Wednesday -- opposition Labour Party has said it will vote with the government, reported BBC.
The Prime Minister needs Parliament's backing to hold a vote before the next scheduled date of 2020.
Explaining her change of heart on an early election, May said: "I have concluded the only way to guarantee certainty and security for years ahead is to hold this election."
She accused Britain's other political parties of "game playing", adding that this risks "our ability to make a success of Brexit and it will cause damaging uncertainty and instability to the country".
"So we need a General Election and we need one now. We have at this moment a one-off chance to get this done.
In a statement outside Number 10, May said: "Labour had threatened to vote against the final Brexit agreement, the Liberal Democrats had stated they wanted to 'grind the business of government to a standstill'."
"... The Scottish Nationalist Party had said they would vote against the negotiations and 'unelected' members of the House of Lords had vowed 'to fight us every step of the way'," she added.
"If we don't hold a General Election now, their political game-playing will continue and the negotiations with the European Union will reach their most difficult stage in the run up to the next General Election," she said.
The Prime Minister challenged the opposition parties: "This is your moment to show you mean it -- to show you're not opposing the government for the sake of it, to show that you do not treat politics as a game."
"Let's tomorrow vote for an election -- let's put forward our plans for Brexit and our alternative programmes for government and then let the people decide."
Labour leader Jeremy Corbyn said he welcomed the Prime Minister's decision, saying it would "give the British people the chance to vote for a government that will put the interests of the majority first".
He said: "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."
In his response to May's announcement, Liberal Democrat leader Tim Farron tweeted: "This is your chance to change the direction of your country.
If you want to avoid a disastrous hard Brexit. If you want to keep Britain in the single market. If you want a Britain that is open, tolerant and united, this is your chance."
"Only the Liberal Democrats can prevent a Conservative majority," he added.
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