Passengers on cross-border trains and buses will be free to ditch face masks in England but legally obliged to put them on as soon as they enter Scotland.
Boris Johnson confirmed his plan on Monday for the legal requirement to wear face masks in England to be scrapped in all settings on July 19.
However, the legal obligation to wear masks on public transport and other settings will remain in force north of the border until at least August 9 – Scotland’s "Freedom Day" – and could stay for much longer.
Nicola Sturgeon, the First Minister, has said there will still be a need for face masks on public transport after August, although she is yet to decide whether this will be written into law.
The divergent regimes on both sides of the border, however, means that for at least three weeks passengers will be able to sit on carriages for hundreds of miles without masks, but will then be breaking the law if their faces are not covered once they pass Gretna or Berwick-upon-Tweed.
Technically, passengers could be fined £60 if they refuse to wear a face mask once they pass into Scotland, although British Transport Police is yet to clarify whether it will enforce the divergent rules.
Graham Simpson, transport spokesman for the Scottish Tories, said the situation must be urgently resolved.
"Passengers who use cross-border trains on a daily basis and our police officers need the SNP Government to urgently clarify what rules on face coverings, if any, will remain in place on these services," he said.
"They must deliver a clear message to ensure that passengers won’t unwittingly be punished if guidance is different in Scotland compared with England.
"That will help maintain confidence in public health messaging going forward as we return to normality."
Professor Jason Leitch, Scotland’s National Clinical Director, said on Monday that he "hoped" a situation would not arise where people would have to put masks on when entering Scotland on trains.
However, Mr Johnson’s statement on Monday effectively confirmed the different rules would be in place, assuming the Prime Minister sticks to his July 19 date.
Under Scotland’s Levels system, it is planned that the whole country will move to Level 0 on July 19, although this means the rules on face masks will remain in force.
"I hope it doesn’t work quite like that,” Prof Leitch told the BBC, when asked whether train passengers would have to put masks on at Berwick if they were travelling north.
“I think people are pretty responsible. I get the train to Edinburgh [from Glasgow] relatively often and it is a very responsible group of travellers.
“As we come back, of course personal responsibility and common sense will have to be more to the fore. The decision about law or guidance will be one for the First Minister and the Cabinet, not for me.
“We will advise that some things in the pandemic will have to stay in place … face coverings would still be a sensible thing to do.”
Ms Sturgeon is still deciding whether there will be a continued legal requirement for face coverings in shops and on public transport, or whether this will become guidance with which the public is requested to comply.
However, sources within the industry said it would not be for staff on trains to enforce the rules, meaning the matter would likely be left to British Transport Police.
The Rail Delivery Group, which represents operators, said that public transport should not be singled out for different rules for masks when they were not mandated in other sectors.
"Trains should be treated consistently with other indoor settings when it comes to the removal or on-going use of restrictions," a spokeswoman said.
"Travelling by train is low risk and carriages are well ventilated with air regularly refreshed either by air conditioning systems, or by doors and windows being opened, so any decision to leave public transport behind other parts of the economy would need to be based on the science.
"Of course, train companies will continue with extra cleaning and better information about how busy services are, and given that wearing a mask helps protect others, we would also support people who wished to continue wearing one in future if it becomes voluntary."
Asked where and when he would wear a mask, Mr Johnson said: "It will depend on the circumstances.
“Clearly there’s a big difference between travelling on a crowded Tube train and sitting late at night in a virtually empty carriage on the main railway line.
"We want people to exercise their personal responsibility but remember the value of face coverings both in protecting themselves and others."