Scotland should hold an independence referendum before the current Scottish parliamentary term ends in May 2021 and will prepare legislation for this to happen, First Minister Nicola Sturgeon said on Wednesday.

"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 Holyrood, Scotland's devolved parliament.

She said a devolved parliament bill would be drawn up before the end of 2019.

The permission of Britain's sovereign parliament at this stage was not needed, she said, but would eventually be necessary "to put beyond doubt or challenge our ability to apply the bill to an independence referendum."

Sturgeon is under pressure from her nationalist movement to provide a clear way forward in the quest for an independent Scotland.

But Britain is mired in political chaos due to Brexit and it is still unclear whether, when or even if Britain will leave the European Union.

Scotland, part of the United Kingdom for more than 300 years, rejected independence by 10 percentage points in a 2014 referendum.

Differences over Brexit have strained the United Kingdom. Scotland and Northern Ireland voted to stay in the EU in a 2016 referendum, while Wales and England vote to leave.

Those who want to maintain the United Kingdom argue that Brexit has made no difference to how Scots feel, and the secession vote should not be repeated.

But Sturgeon argued that leaving the world's largest trading bloc endangers Britain and Scotland's economic well-being.

"We face being forced to the margins, sidelined within a UK that is itself increasingly sidelined on the international stage. Independence, by contrast, would allow us to protect our place in Europe," she said. "We need a more solid foundation on which to build our future as a country."