Scotland's First Minister Nicola Sturgeon said the Scottish parliament would publish an independence referendum bill with a planned date for a vote on secession to take place on 19 October, 2023.
Ms Sturgeon said she would be writing to British Prime Minister Boris Johnson for permission to hold a consultative referendum, saying it was vital the vote was legal.
She said the issue would be referred to the UK Supreme Court.
Voters in Scotland, which has a population of around 5.5 million, rejected independence in 2014.
But the Scottish government says Britain's departure from the EU, which was opposed by a majority of Scots, means the question must be put to a second vote.