Lawmakers in Edinburgh are due to vote on Scottish leader Nicola Sturgeon's bid for a new referendum, despite the prime minister's last-minute appeals.
The Scottish vote had been scheduled for last Wednesday but was postponed after the terror attack near the British parliament in London, the same day, in which four people were killed and dozens more injured.
The attack has not, however, put the brakes on Britain's EU divorce, with May vowing to stick to her timetable of invoking Article 50 of the European Union's Lisbon Treaty on Wednesday and thus triggering two years of Brexit negotiations.
The Brexit vote last year has spurred the independence campaign of Sturgeon, head of the ruling Scottish National Party (SNP), who argues that Scotland is being forced out of the European bloc against its will.
Both Scotland and Northern Ireland voted to remain in the EU, but they were outnumbered by voters in England and Wales who backed Brexit.