Scotland will start preparing for an independence referendum before May 2021 without permission from London, First Minister Nicola Sturgeon says.
Scotland, part of the United Kingdom for more than 300 years, rejected independence by 10 percentage points in a 2014 referendum.
But differences over Brexit have strained relations with England and the British government in London.
"A choice between Brexit and a future for Scotland as an independent European nation should be offered in the lifetime of this parliament," Sturgeon told Scotland's devolved parliament on Wednesday.
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She said a devolved parliament Bill would be drawn up before the end of 2019, and that Scotland did not need permission at this stage from London.
London's approval, however, would eventually be necessary "to put beyond doubt or challenge our ability to apply the Bill to an independence referendum", she said.
The UK voted 52-48 to leave the EU in a 2016 referendum, but while Wales and England voted to leave, Scotland and Northern Ireland voted to stay.
A spokesman for British Prime Minister Theresa May said the results of the 2014 referendum should be respected.