The Scottish National Party (SNP) published a "Roadmap to Referendum" on Saturday, laying out plans for another vote on Scottish independence just as the UK grapples with COVID-19 and the impact of Brexit.

An anti-Brexit pro-Scottish independence activist holds a flag mixing the EU flag and the Scottish Saltire as she gathers for a small protest against Britain's exit from the European Union outside the Scottish Parliament in Edinburgh on December 31, 2020. (Photo: AFP)

British Prime Minister Boris Johnson, who would have to agree to a new referendum, argues there is no need for a new vote after independence was rejected by Scottish voters in 2014.

But the SNP has said that if it wins a parliamentary majority at elections scheduled for May 6, it will pass its own bill so that a referendum can take place once the pandemic is over.

It says that the UK government will either have to agree to it or take legal action to dispute the legal basis of the referendum.

"Such a legal challenge would be vigorously opposed by an SNP Scottish government," said the document on Sunday.

Polls show that Scottish nationalists are on course to win a record majority in elections for Scotland's devolved parliament.

A poll published on The Times website on Saturday showed another potential threat to the future of the UK, with 51 percent of Northern Irish voters saying they favored a referendum about their future within the next five years, compared to 44 percent who oppose it.

Support for a United Ireland stands at 42 percent according to the polls, versus 47 percent who object to Northern Ireland leaving the UK.

The bonds holding the UK together have been severely strained over the last five years by Brexit, the government's handling of the COVID-19 pandemic, and repeated calls by the SNP for a new referendum on independence.

Scottish voters rejected independence by 55 to 45 percent in a 2014 referendum, but since then the SNP have become stronger, winning all elections in Scotland by huge margins.

They are expected to perform strongly in elections to the Scottish parliament in May.

Pro-EU supporters hold placards and banners during a 'Yes to Europe' rally in London's Trafalgar square, ahead of EU referendum, in central London on June 21, 2016. (Photo: AFP)

In the 2016 Brexit referendum, England and Wales voted to leave the European Union but Scotland and Northern Ireland voted to remain.

Due to the much greater size of England's population, the overall result was a win for Brexit.

This was galling for many Scots as one of the central arguments put forward in favor of remaining in the UK in their own 2014 referendum was that it was the only way for Scotland to remain part of the European bloc.

Johnson in 2020 called the devolution of powers to Scotland "a disaster," and said he saw no case for giving Scotland's semi-autonomous government and parliament any further powers in addition to the ones they hold now.

SNP lawmaker Drew Henry said Johnson's comments showed his and his party's "contempt" for the people of Scotland.