Scotland's devolved parliament is set to vote to give its government a mandate to seek a new independence referendum, despite Prime Minister Theresa May saying "now is not the time" for another vote as complex EU exit negotiations loom.
London's permission for a new Scottish referendum is needed because any legally binding vote on United Kingdom constitutional matters has to be authorised by the UK parliament.
May has not completely ruled out another Scottish independence vote but has vowed to fight for what she has called the "precious union" of the United Kingdom.
Dealing with Scotland simultaneously with Brexit could weaken her hand, according to a UK government source.
First Minister Nicola Sturgeon, who argues that Scotland's vote to keep its EU membership in last June's referendum has been ignored in May's Brexit arrangements so far, is seeking authority for a second referendum from the Scottish parliament, to be held in late 2018 or early 2019.
"As a result of the Brexit vote we know that change is now inevitable - the question is what kind of change is right for Scotland and whether that change is made for us or by us," Ms Sturgeon told parliament as she opened a two-day debate which ends with Wednesday's vote.
The current balance of power in the Scottish parliament means Ms Sturgeon is almost certain to win its backing.
Conservatives, Labour and the Liberal Democrats, opposition parties in Scotland's parliament, say Ms Sturgeon is using Brexit as an excuse to raise the issue of independence again after Scots rejected secession by a 10-point margin in 2014.
The Scottish leader could hold a non-binding vote on independence to prove her support, or could even resign to force a snap election in hopes of securing a bigger mandate.