EDINBURGH (AFP, BLOOMBERG, REUTERS) - Scottish First Minister Nicola Sturgeon yesterday demanded a new Scottish independence referendum to be held late next year or early 2019, once the terms of Britain's exit from the European Union (EU) have become clear.

She said she will seek the authority of the Scottish Parliament to agree with the British government on a so-called Section 30 Order - granting "the ability of Scotland to legislate for an independence referendum".

"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," Ms Sturgeon, who heads the Scottish National Party (SNP), told reporters.

Her announcement came as British Prime Minister Theresa May was set this week to trigger the process of leaving the EU after last year's Brexit vote.

Mrs May yesterday chided Ms Sturgeon for the demand for a referendum, saying the SNP had "tunnel vision" on breaking away from the United Kingdom.

"The tunnel vision that SNP has shown today is deeply regrettable," Mrs May said.

"Instead of playing politics with the future of our country, the Scottish government should focus on delivering good government and public services for the people of Scotland. Politics is not a game," she added.

Scotland rejected independence by 55 per cent in a September 2014 vote and the British government said that settled the question.

Ms Sturgeon and her supporters argue that the Brexit vote changed all that since Scotland voted for Britain to stay in the EU by 62 per cent in the referendum in June last year.

Ultimately, it is the British Parliament in Westminster - where Mrs May commands a majority - which makes the call on whether Scotland can hold a second referendum.

But if Mrs May refuses to approve such a vote, she could provoke a constitutional crisis.

A BMG survey of 1,009 people for Scottish broadsheet The Herald found that 56 per cent of those who expressed views were against another independence vote before Brexit occurs.

Some 52 per cent said they were against Scotland seceding from Britain. But an Ipsos Mori poll showed support for independence on a knife-edge, with 49 per cent of Scots in favour.

The leader of Scotland's Conservative Party said yesterday Ms Sturgeon's call for a second independence referendum was divisive and pledged to vote against her request.

"Nicola Sturgeon has today chosen the path of further division and uncertainty," Ms Ruth Davidson, whose party is the second-largest in the Scottish Parliament, said on Twitter.

She added that she would vote against any legal move to hold a referendum.