In a major blow to the First Minister, the European Commission made it clear that a separate Scotland would have to apply from scratch for membership. This means it would be outside both the UK and the EU for an indeterminate period.
Ruth Davidson, the Scottish Conservative leader, said Sturgeon was being "utterly irresponsible" and "has today given up acting as First Minister for all of Scotland".
The First Minister's denials that she had been planning a second independence referendum, regardless of the UK Government's stance, were undermined by the Nationalists launching a website to coincide with her speech. It included a video from Sturgeon and a fundraising target of £1 million within 100 days.
She made the announcement only hours after another poll showed a majority of Scots did not want another referendum before Brexit takes place in March 2019.
The timing of the announcement today - which was revealed by the Telegraph - came just 24 hours before Theresa May, the British Prime Minister, is expected to trigger the start of the process of leaving the EU.
Since the UK vote to leave the European Union, Sturgeon has repeatedly said a second independence referendum is "highly likely".
Scots voted 62 per cent to 38 per cent to remain the EU while the UK as a whole voted to leave.
The SNP's manifesto for last year's Holyrood election states that the party will seek a second referendum if there is a "significant material change" in Scotland's position at the time of the last referendum in 2014, such as "Scotland being taken out of the EU against our will".
However , the First Minister also promised at the time that she would not hold another referendum unless support for independence increased significantly. This has not happened.
Speaking at a specially-arranged press conference at her Bute House official residence in Edinburgh, Sturgeon accused the UK Government of "intransigence" over its refusal to agree her complex plan for Scotland to stay in the single market when the rest of the UK comes out.
The First Minister said: "I will now take the steps necessary to make sure that Scotland will have a choice at the end of this process.
"A choice of whether to follow the UK to a hard Brexit - or to become an independent country, able to secure a real partnership of equals with the rest of the UK and our own relationship with Europe."
She added: "So I can confirm today that next week I will seek the approval of the Scottish Parliament to agree with the UK government the details of a Section 30 order - the procedure that will enable the Scottish Parliament to legislate for an independence referendum."
Sturgeon argued that the Scottish Parliament - which has a nationalist majority when SNP and Green MSPs are combined - must have control over the referendum's timing.
She said that autumn next year would be the earliest "appropriate" time as only then will be the shape of the Brexit deal be known, but she also argued that a second independence referendum could not be held long after the UK leaves the EU in March 2019.
"If the UK leaves the EU without Scotland indicating beforehand - or at least within a short time after it - that we want a different relationship, we could face a lengthy period outside not just the EU but also the single market. That could make the task of negotiating a different future much more difficult," she said.
"These considerations lead me to the conclusion that if Scotland is to have a real choice - when the terms of Brexit are known, but before it is too late to choose our own course - then that choice must be offered between the autumn of next year, 2018, and the spring of 2019."
Asked what recourse she would have if May refused to allow a referendum before Brexit, Sturgeon said: "Having sunk the ship with the Brexit vote, that would be puncturing Scotland's lifeboat as well and I don't think that would be acceptable."
Sturgeon denied she had been planning a second referendum all along, having ordered her official to draw up legislation for another vote only hours after the Brexit result became known on June 25 last year.
Instead, she argued that May had forced her into calling another independence vote by refusing to agree to her "compromise" plan for Scotland to stay in the single market. This would have involved devolving all powers except defence and the macro-economy.
The First Minister was clearly annoyed that she has not been told when Article 50 will be triggered, which showed the sort of relationship Scotland had with Westminster.
She said: "I don't know whether Article 50 is going to be triggered tomorrow, or Wednesday, or next week, or the week after that."
Sturgeon rejected accusations she is failing to respect the result of the 2014 referendum, which the Unionists won by a margin of 55 per cent to 45 per cent, arguing there may have been a "different decision" if voters had known about Brexit. She added: "What is at stake is the kind of country we will become."
Amid fierce criticism of the honesty of the case the Nationalists presented in 2014, when she predicted a second oil boom, she promised that Scots would get an informed debate. However, she refused to answer specific questions about the currency or how Scotland's mammoth deficit would be dealt with.
A UK Government spokesman said: "As the Prime Minister has set out, the UK Government seeks a future partnership with the EU that works for the whole of the United Kingdom. The UK Government will negotiate that agreement, but we will do so taking into account the interests of all of the nations of the UK.
"We have been working closely with all the devolved administrations - listening to their proposals, and recognising the many areas of common ground, including workers' rights, the status of EU citizens living in the UK and our security from crime and terrorism.
"Only a little over two years ago people in Scotland voted decisively to remain part of our United Kingdom in a referendum which the Scottish Government defined as a 'once in a generation' vote.
"The evidence clearly shows that a majority of people in Scotland do not want a second independence referendum.
"Another referendum would be divisive and cause huge economic uncertainty at the worst possible time. The Scottish Government should focus on delivering good government and public services for the people in Scotland."
The European Commission indicated an independent Scotland would have to apply to join the EU, rather than automatically being a member. Commission spokesman Margaritis Schinas said the "Barroso doctrine" continued to apply.
Former commission president Jose Manuel Barroso set out the legal view that if one part of an EU country became an independent state it would have to apply for EU membership.
At a briefing in Brussels, Schinas said: "The commission does not comment on issues that pertain to the internal legal and constitutional order of our member states." But he added: "The Barroso doctrine, would that apply? Yes that would apply, obviously."
