Prime Minister Theresa May has made a "huge political miscalculation" in calling a snap election, Nicola Sturgeon warned, as she said the vote could strengthen the case for a second Scottish independence referendum.

The First Minister is convinced she already has a "cast-iron mandate" to hold another vote on the issue - but said that the June 8 election could help with "reinforcing the democratic mandate which already exists for giving the people of Scotland a choice on their future".

Ms Sturgeon also said the "extraordinary U-turn" from Mrs May would give voters north of the border a further chance to reject the "narrow, divisive" policies of the Tories.

She spoke out after Mrs May stunned Westminster with her announcement - which came after repeated denials from Downing Street that there would be a snap vote.

The Conservative leader said she would be "campaigning for the future of the United Kingdom" in the June 8 election, and the Tories - who have seen support grow in Scotland under Ruth Davidson - will highlight their opposition to a second vote on independence.

Mrs May told ITV News: "I will be out there championing the cause of a United Kingdom. I believe that we are stronger as a United Kingdom. I think this Union we have between Northern Ireland, Scotland, England and Wales is a very precious Union.

"I think we all benefit by it. And I will unashamedly be out there campaigning for the future of the United Kingdom."

Ms Sturgeon argued the upcoming election "will be about the kind of country we want Scotland to be".

The campaign will take place at the same time as Ms Sturgeon's SNP Government at Holyrood is pushing for a fresh vote on independence to be held sometime between autumn 2018 and spring 2019 - with ministers insisting this is necessary to give Scots a choice between leaving the UK and Brexit.

The Prime Minister had been adamant that "now is not the time" for such a vote to be held - but a spokeswoman for Ms Sturgeon the Tory leader "has kind of shot her own fox with that" by calling a snap election.

The First Minister meanwhile accused the PM of putting her party before the country, saying Mrs May "is clearly betting that the Tories can win a bigger majority in England given the utter disarray in the Labour Party".

The SNP leader stated: "That makes it all the more 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."

She later told the BBC: "I've got a mandate for a second referendum; I won that mandate at the Scottish Parliament elections last year and of course the Scottish Parliament has since voted by majority for that position, so that mandate is there and it is clear.

"This election will be about the kind of country we want Scotland to be and whether we want the Tories to have a free hand in determining that or whether we want to make sure that we stand up for Scotland's public services, for public spending, against further Tory austerity."

Ms Sturgeon's spokeswoman said the SNP had been clear since Holyrood was established that "the mandate for a referendum comes from the Scottish Parliamentary elections".

She added: "We already have a cast-iron mandate for having a referendum and giving the people of Scotland the right to choose - we have that from the 2016 elections, from the outcome of the Brexit referendum, from the vote in Parliament.

"Undoubtedly this will be an issue in the referendum, but the position is one that has been set out.

"I think the Prime Minister might have miscalculated if she thinks that having this is going to give her a clear mandate in Scotland against a referendum."

The First Minister has yet to set out the "next steps" in her plans for an independence vote, with her spokeswoman stating the announcement of an election in just a few weeks' time "may affect the timing in a short term sense" for that update.

In the 2015 general election the SNP picked up 56 of the 59 constituencies north of the border - a result they could struggle to repeat.

In contrast, Labour, the Conservatives and the Liberal Democrats won just one seat each.

Ms Davidson said: "The Scottish Conservatives are ready for a campaign, we are organised, and we are optimistic about the prospect of increasing our number of seats.

"In Scotland, we will have a clear election message - only a vote for the Scottish Conservatives will ensure we get the strong leadership we need to get the best Brexit deal for the whole country.

"And only a vote for the Scottish Conservatives will send a strong message that we oppose the SNP's divisive plan for a second referendum."

She claimed the SNP will "use this campaign to try and manufacture a case for separation", but insisted: "The Scottish Conservatives have the strength right across Scotland to stand up for people who oppose the SNP's plans."

Scottish Labour leader Kezia Dugdale said the upcoming general election would give voters a ''significant and historic choice''.

She stated: "At this election, the choice again will be clear: a Tory Party intent on a hard and damaging Brexit; or a Labour Party that will oppose a second independence referendum and fight for a better future for everybody.''

Ms Dugdale added: ''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.''

Scottish Liberal Democrat leader Willie Rennie also said his party was ''relishing the prospect of this election''.

He stated: ''It is a chance to change the direction of the whole of the UK. Our optimistic agenda is pro-UK, pro-EU and progressive. We stand with the majority opinion in this country.

''At this election we will stand proud for a United Kingdom within the European single market. We will avoid a disastrous hard Brexit and keep Britain united. Liberal Democrats the are only party that can stop a Conservative majority across the UK."

Meanwhile Scottish Green co-convener Patrick Harvie said Scotland's opposition to leaving the EU "cannot be allowed to fall on deaf ears" in the election.

He said: "I'm sure that voters in Scotland will not allow this election to satisfy the whims of a dangerous hard-right party determined to damage our economy, public services and threaten our EU friends and neighbours who have made Scotland their home. Instead, we must reassert our choice for a fairer and more equal society."