May said she had already set out her plans for Britain's departure from the European Union in a document published earlier in the year.
The UK prime minister formally triggered Article 50 of the Lisbon Treaty and European Union officials say they do not expect the election to make any difference to the timing of talks.
Prior to May's announcement on Tuesday morning, the pound had slumped somewhat, but recovered well following the call for an election.
"Brexit negotiations will start after 8 June elections", said Juncker to May, according to the Commission chief spokesperson Margaritis Schinas.
Under British electoral law, May needs the backing of two-thirds of lawmakers.
Mrs May's extraordinary move came after national polls suggested the Conservatives had a 21-point advantage over Labour - the biggest lead for any party in government since 1983. "Let us put forward our plans for Brexit and our alternative programs for government and then let the people decide", May said.
The announcement has also emboldened independence activists in Scotland and Northern Ireland.
Shadow chancellor John McDonnell dismissed the polls that showed Labour lagging far behind and accused Mrs May of breaking the public's trust over her volte face on a snap election. British voters are set to cast their ballots for the House of Commons' 650 seats on June 8. Brexit will likely dominate the campaign agenda, with many perceiving the election as a vote on May's Brexit leadership.
With polls in the United Kingdom showing the government and its negotiating position is popular, while the opposition party, much like the KMT, is stuck in internal disarray and polling at historic lows, the Prime Minister has made the political judgment that she can win a bigger majority and a mandate for her negotiating position through a general election.
May's governing Conservatives now have a slight majority, with 330 seats in the 650-seat House of Commons.
Theresa May won't take part in any TV debates in the run-up to the general election in June, Downing Street said.More news: Man sought in Cleveland Facebook killing is dead
"However, we believe that the Prime Minister's decision to call this election is a cynical decision driven more by the weakness of Corbyn's Labour Party rather than the good of the country". Once again, the Tories are putting party-political interests ahead of the national interest. Corbyn, a traditional labourist, will campaign on reversing government austerity, nationalizing railways, and investing in wages, all while steering clear of Brexit so as to not alienate the party's pro-leavers. But if her victory is anything less than overwhelming, May's snap election could end up dramatically weakening her authority, which would be bad news for her Brexit ambitions.
Mr Corbyn said the election "gives the British people the chance to vote for a Labour government that will put the interests of the majority first".
Granted, the Liberal Democrat leader, Tim Farron, does not look particularly prime ministerial.
The Telegraph reflected the nation's surprise at the imminent election: "May's bolt from the blue".
What Does Theresa May Hope to Accomplish with Snap Elections?
"Fortuitously for them, they find themselves in a position of being the only party that is making the anti-Brexit case", said Rob Ford, political science professor at the University of Manchester.
Ms May insisted an early election would provide "certainty and stability" in the negotiations, which will now start after the vote. Ukip and the Lib Dems were both on 10pc.
Scotland's First Minister also told Sky News she was open to a "progressive alliance" with Labour and the Liberal Democrats to keep the Tories out if the "parliamentary arithmetic" added up.
"My sense is that a stronger mandate and more time would allow a more patient approach and a softer Brexit, probably more in line with May's instincts".
Last month, May formally began the two-year divorce talks with the European Union and laid out her hope that her government can settle the exit terms alongside talks on what the new relationship with the European Union will be.
May is hoping to gain a bigger majority in Parliament for her Conservatives, strengthening her negotiating hand with the EU. However, the Euroskeptic party still has a support base, and leader Paul Nuttall will seek to paint May as a political opportunist. She could claim her mandate to implement a hard Brexit before the consequences hit home, while the economy is still riding high on debt-fuelled consumption. The government's current working majority is only 17 seats.