Theresa May has called a snap general election on June 8, claiming that divisions at Westminster risked hampering the Brexit negotiations.
The Prime Minister will require the support of two-thirds of MPs to go to the country, with a vote scheduled in the Commons on Wednesday.
The move stunned Westminster, as Mrs May and Number 10 have repeatedly insisted she would not seek a general election before the scheduled 2020 poll.
But Mrs May, who has a fragile working majority of just 17 in the Commons, said she wanted "unity" at Westminster as talks on Brexit begin in earnest with the European Union.
She said: "We want a deep and special partnership between a strong and successful European Union and a United Kingdom that is free to chart its own way in the world.
"That means we will regain control of our own money, our own laws and our own borders and we will be free to strike trade deals with old friends and new partners all around the world.
"This is the right approach, and it is in the national interest. But the other political parties oppose it.
"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 acknowledged that she needed a stronger position in the Commons to secure her plans for the UK's future outside the EU.
"Our opponents believe because the Government's majority is so small that our resolve will weaken and that they can force us to change. They are wrong," she said.
"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, because what they are doing jeopardises the work we must do to prepare for Brexit at home and it weakens the Government's negotiating position in Europe."
Under the Fixed-Term Parliaments Act, general elections take place every five years, meaning Mrs May would have had to carry on until 2020 before the chance to strengthen her position.
In order to call the early election, she will need the support of two-thirds of the 650 MPs in the Commons - but Labour is expected to support her, as any opposition would look weak if it did not agree to the chance to take office.
Senior Tories have urged Mrs May to call an early election, taking advantage of the Conservatives' healthy opinion poll lead over Jeremy Corbyn's Labour.
Mrs May suggested she reached her decision over the Easter parliamentary recess - during which she went on a walking holiday in North Wales.
"I have only recently and reluctantly come to this conclusion," the PM said.
"Since I became Prime Minister I have said that there should be no election until 2020.
"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."
Labour leader Jeremy Corbyn welcomed the news.
He said in a statement: "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.
"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 the last couple of weeks, Labour has set out policies that offer a clear and credible choice for the country. We look forward to showing how Labour will stand up for the people of Britain."
Mrs May spoke to the Queen on the phone on Easter Monday before announcing the early vote on Tuesday, the Prime Minister's official spokesman said.
The PM got the full backing of the Cabinet before calling the election.
MPs are being contacted for a comment.