The ruling Scottish National Party on Thursday pledged to hold an independence referendum by the end of 2023, which could rip apart the 314-year union between England and Scotland.
If there were another referendum and the Scots voted to go it alone, it would be the biggest shock to the UK since Irish independence a century ago, just as London tries to deal with the effects of Brexit and the Covid-19 crisis.
Scotland voted against independence by 55 per cent to 45 per cent in 2014, but First Minister Nicola Sturgeon's ruling party wants another referendum if it wins the Scottish Parliament election on May 6.
British Prime Minister Boris Johnson has repeatedly dismissed the party's calls for a referendum.
"We believe the people of Scotland should have the opportunity in a referendum when the Covid crisis is over to decide whether Scotland should be an independent country," the SNP said in its manifesto published on Thursday.
"The SNP intention is for the referendum to be within the first half of the five-year term."
The party said the date of the referendum would be "determined by our democratically elected parliament".
Mr Johnson has the power to refuse another independence vote but he would probably face a constitutional showdown with Scotland.
The SNP proposed that the question on the ballot could be: "Should Scotland be an independent country?"
"There can be no moral or democratic justification for Boris Johnson or any Westminster government to obstruct the right of the people of Scotland to decide their own future," it said.
The SNP has 61 of the 129 seats in the Scottish Parliament but by working with the Greens' five ministers, it has effective control.
The nations of Britain have shared the same monarch since James VI of Scotland became James I of England in 1603 and a formal union created the kingdom of Great Britain in 1707.
But Brexit and Mr Johnson's handling of the Covid-19 crisis have strained the ties that bind together England, Northern Ireland, Scotland and Wales into the UK.
Britain voted 52 per cent to 48 per cent to leave the EU in a 2016 referendum.
England and Wales voted to leave but Scotland and Northern Ireland voted to stay.