British Prime Minister Theresa May has called for an early election - on June 8 - saying she needs a fresh mandate to move ahead with Britain's planned break from the European Union.

The surprise announcement, made outside her office at 10 Downing Street, comes amid political strains over Brexit and moves by Scotland to possible carve its own independent path to remain in the European Union.

"I have concluded 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 have to make," she said.

Last month, Britain submitted its formal request to begin EU exit negotiations.

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Theresa May became prime minister in July last year, shortly after the referendum that set in motion Britain's EU divorce and prompted the resignation of her predecessor, David Cameron.

May also is taking a political gambit.

If her Conservative party wins the snap elections, she will have the mandate to pursue her own agenda as she heads into negotiations as Britain exits the European Union - not the one set out by her predecessor Cameron.

But a loss would throw deep uncertainly into the complex talks ahead between Britain and the other 27 EU members, and reflect important shifts in the British sentiment since last year's referendum.

Polls show the Conservative party has a strong lead of more than 20 points over the opposition Labour party. But the announcement still caught some off guard because Downing Street has repeatedly denied May would seek an early election. The next one was scheduled for 2020.

The British prime minister used to be able to "call" an election when he or she wanted. But the 2011 Fixed-Term Parliaments Act changed that, setting the date of the election every five years, meaning the next election was scheduled for May 2020.

But the leader can still seek an early election if two thirds of the lawmakers in the House of Commons vote for it. Jeremy Corbyn, the leader of the opposition Labour, has previously said he would agree to an early election.

It is rare for leaders to make statement outside No. 10 Downing Street - it's usually reserved as the setting for major announcements.

"We need a general election, and we need one now," May said, "because 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."

Although Britain as a whole voted 52 to 48 percent in favor of leaving of the European Union, majorities in both Scotland and Northern Ireland favored staying in the EU.

Scottish leader Nicola Sturgeon has charged that Scottish voters are being taken out of the bloc against their will, and said last week that she wants a referendum on independence - a rerun of a September 2014 vote, in which a majority of Scottish voters opted to stay in the United Kingdom - between the autumn of 2018 and the spring of 2019.

May has sharply criticised that call. She said at the weekend that "now is not the time" for a Scottish vote. But she has not threatened to veto another referendum.

May announces the snap election. Photo / AP
May announces the snap election. Photo / AP

British Prime Minister's statement

'I have just chaired a meeting of the Cabinet, where we agreed that the Government should call a general election, to be held on June 8.

'I want to explain the reasons for that decision, what will happen next and the choice facing the British people when you come to vote in this election.

'Last summer, after the country voted to leave the European Union, Britain needed certainty, stability and strong leadership, and since I became Prime Minister the Government has delivered precisely that.

'Despite predictions of immediate financial and economic danger, since the referendum we have seen consumer confidence remain high, record numbers of jobs, and economic growth that has exceeded all expectations.

'We have also delivered on the mandate that we were handed by the referendum result.'

'Britain is leaving the European Union and there can be no turning back. And as we look to the future, the Government has the right plan for negotiating our new relationship with Europe.

'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.

'In recent weeks Labour has threatened to vote against the deal we reach with the European Union.

'The Liberal Democrats have said they want to grind the business of government to a standstill.

Britain's Prime Minister Theresa May, right, and Scotland's First Minister Nicola Sturgeon sit during their meeting in Glasgow, Scotland. Photo / AP
Britain's Prime Minister Theresa May, right, and Scotland's First Minister Nicola Sturgeon sit during their meeting in Glasgow, Scotland. Photo / AP

'The Scottish National Party say they will vote against the legislation that formally repeals Britain's membership of the European Union.

'And unelected members of the House of Lords have vowed to fight us every step of the way.

'Our opponents believe that because the Government's majority is so small, our resolve will weaken and that they can force us to change course.

'They are wrong.

'They under-estimate 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.

'If we do not 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 scheduled election.

'Division in Westminster will risk 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, because 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.

'I have only recently and reluctantly come to this conclusion.

'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.

'And so tomorrow I will move a motion in the House of Commons calling for a general election to be held on the eighth of June.

British opposition Labour Party leader Jeremy Corbyn. Photo / AAP
British opposition Labour Party leader Jeremy Corbyn. Photo / AAP

'That motion, as set out by the Fixed-Term Parliaments Act, will require a two-thirds majority of the House of Commons.

'So I have a simple challenge to the opposition parties, 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.

'This is your moment to show you mean it, to show you are not opposing the Government for the sake of it, to show that you do not treat politics as a game.

'Let us tomorrow vote for an election, let us put forward our plans for Brexit and our alternative programmes for government and then let the people decide.

'And the decision facing the country will be all about leadership. It will be a choice between strong and stable leadership in the national interest, with me as your Prime Minister, or weak and unstable coalition government, led by Jeremy Corbyn, propped up by the Liberal Democrats - who want to reopen the divisions of the referendum - and Nicola Sturgeon and the SNP.

'Every vote for the Conservatives will make it harder for opposition politicians who want to stop me from getting the job done.

'Every vote for the Conservatives will make me stronger when I negotiate for Britain with the prime ministers, presidents and chancellors of the European Union.

'Every vote for the Conservatives means we can stick to our plan for a stronger Britain and take the right long-term decisions for a more secure future.

'It was with reluctance that I decided the country needs this election, but it is with strong conviction that I say it is necessary to secure the strong and stable leadership the country needs to see us through Brexit and beyond.

'So, tomorrow, let the House of Commons vote for an election, let everybody put forward their proposals for Brexit and their programmes for Government, and let us remove the risk of uncertainty and instability and continue to give the country the strong and stable leadership it demands.'