First Minister Nicola Sturgeon has insisted that the level system has “until now, been effective” in Scotland.
But she said that case numbers in the country had risen by 15 per cent in the past couple of weeks.
Shetland has been in level one since the beginning of November, and only recorded two coronavirus cases in December, but will move into level three from Boxing Day.
Ms Sturgeon said that information in the past eight days about the new strain of the virus had meant they needed to act quickly.
If they did not act, and waited to find out more about the new strain, it would be “too late”.
She said the virus “may spread much faster now than in March”.
“I do not expect a single person to be happy” about the new restrictions, she said, but she added they were “necessary” and “essential”.