Nicola Sturgeon is facing questions over draft regulations that make it illegal to enter or leave Scotland without a reasonable excuse.
The restrictions, brought in to deal with the coronavirus crisis, affect England, Wales and Northern Ireland as well as the Republic of Ireland, which is part of the Common Travel Area.
Tough rules on travel within Scotland will also come into force at 6pm.
The ban on both cross-border and internal travel is tempered by what the Scottish government has described as exemptions.
These can include a driving test and donating blood, as well as for things like health, work and education.
A holiday is not a reasonable reason to break the rules, published guidance warns.
Many people near the border cross between England and Scotland daily, or sometimes multiple times a day, to travel to jobs, school or even to shop.
Scottish Conservative MSP and professor of public law Adam Tomkins questioned if Ms Sturgeon’s government had the power to introduce such restrictions.
He said: “Is this within Holyrood’s competence? For one thing, freedom of movement would appear to be expressly reserved to the UK parliament under the Scotland Act. For another, it’s not clear that the Scottish parliament can make rules contrary to the common travel area, as agreed to by the UK and Ireland.
“It’s not at all clear if the draft regulations published today are within the remit of the Scottish parliament.”
A Scottish government spokesperson insisted the regulations were “entirely within the remit of the Scottish parliament”.
“Restrictions on unnecessary travel are in place in law in various forms in England, Wales, Northern Ireland, Ireland and in many parts of Europe.
“In Scotland they are necessary to underpin an approach that puts different parts of the country under different levels of protective measures.
“Travel restrictions are difficult, but essential. If people don’t abide by the travel restrictions there is a risk that the virus will spread to areas where it is less common and we may have to return to national restrictions.”
