On a trip to Scotland to try to stem growing support for another referendum, Johnson opted for a blunt message, saying independence supporters had their chance in 2014 in a vote they had agreed at the time was "a once-in-a-generation event".

The bonds that tie England, Wales, Scotland and Northern Ireland together in a $3 trillion economy have been severely strained by both Britain's exit from the European Union and Johnson's handling of the coronavirus outbreak.

Opinion surveys indicate a majority of Scots would now favour breaking apart the 314-year-old union between England and Scotland.

But Johnson, whose unpopularity runs deep in Scotland according to opinion polls, suggested he was sticking to his position of not approving another referendum, which the Scottish National Party needs to hold a legal vote.