The Tory leader will ask Parliament to unpick the fixed terms election act, allowing her to go to the country in just 50 days time.
Nicola Sturgeon called it a “huge political miscalculation by the Prime Minister”.
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It also marks something of a u-turn for May. On at least five separate occasions she has absolutely completely ruled out holding an early general election.
Nevertheless, much to the surprise of her own MPs, the opposition and the press, after the first Cabinet since the Easter break, May stood on the steps of Number 10, and asked the country to unite behind her vision for Brexit.
"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,” she said.
"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 added that her opponents under-estimated the UK government’s “determination to get the job done”. With a working majority of just 17 in the Commons, May faced a tricky time over the next two years of Brexit negotiations.
The Prime Minister needs the support of two-thirds of the 650 MPs in the Commons to change the law and allow a snap election. That will be little problem, with Labour, the SNP and the Liberal Democrats all ready to back the early general election.
May added that she had only “recently and reluctantly” decided to hold the election. It was, she insisted, “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."
The First Minister said May’s announcement was “one of the most extraordinary U-turns in recent political history” and accused the Prime Minister of “once again putting the interests of her party ahead of those of the country.”
In a statement, Sturgeon said: “She is clearly betting that the Tories can win a bigger majority in England given the utter disarray in the Labour Party.
“That makes it all the important that Scotland is protected from a Tory Party which now sees the chance of grabbing control of government for many years to come and moving the UK further to the right – forcing through a hard Brexit and imposing deeper cuts in the process.
“That means that this will be – more than ever before – an election about standing up for Scotland, in the face of a right-wing, austerity obsessed Tory government with no mandate in Scotland but which now thinks it can do whatever it wants and get away with it.
“In terms of Scotland, this move is a huge political miscalculation by the Prime Minister.
“It will once again give people the opportunity to reject the Tories’ narrow, divisive agenda, as well as reinforcing the democratic mandate which already exists for giving the people of Scotland a choice on their future.”
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."
Polls haven’t been kind to Corbyn and his party recently.
Over the weekend, two UK wide separate polls gave the Tories a lead of more than 20 points. According to the most up to date analysis on the Electoral Calculus website, this would see the Tories win 381 seats to Labour’s 182. They suggest the Liberal Democrats and the SNP would win 8 and 56 seats respectively, giving the PM a majority of 112.
Scottish Labour leader Kezia Dugdale, who was launching her party’s local election campaign when the Prime Minister walked out of number 10 said: “"The Labour Party is ready and has been preparing for a General Election. We will start the process of selecting our candidates this afternoon.
"We will work tirelessly to elect Jeremy Corbyn Prime Minister and deliver a Labour Government."
“Only a vote for the Scottish Conservatives will send a strong message that we oppose SNP's divisive plan for a second referendum,” Davidson warned.
She added: "We know the SNP will use this campaign to try and manufacture a case for separation. And with Jeremy Corbyn having already said he is ‘absolutely fine’ with an immediate referendum, we also know that Labour can’t be relied to stand up to them.”
More to follow...