International travelers must give a negative test before departure taken two days before departure, the Scottish Government announced at.
They will also need a negative PCR test on or before day two of arrival, according to measures agreed on a UK four-nation basis, a spokesman said.
In addition, Nigeria has been added to the red list for international travel from 1 p.m.
It will join ten African countries on the red list: Angola, Botswana, Eswatini, Lesotho, Malawi, Mozambique, Namibia, South Africa, Zambia and Zimbabwe.
Transport Secretary Michael Matheson said: “It is important that we take steps now to keep people safe, protect the rollout of the booster program and reduce the chances of unsustainable pressure being put on the NHS during the winter.
“We have always said that it may be necessary to rapidly implement new measures to protect public health in Scotland, in particular with regard to international travel, and these restrictions are proportionate and necessary in relation to this objective.
“We fully understand the impact the changes will have on staff and businesses in the travel and aviation sector, especially as the new variant came at a time when we were beginning to see some signs of improvement. We will not keep the restrictions in place any longer than necessary.”
British Transport Secretary Grant Shapps tweeted: “As researchers work to understand the new Omicron variant, we need to exercise further caution until the picture is clearer.
“We appreciate that this will be difficult for the travel sector as we prioritize public health and protect the progress of our world-leading wax & booster program.”
The latest tightening of travel restrictions follows pressure from Labor and scientists following the introduction of PCR tests this week, but only within two days of arrival.
Shadow Home Secretary Yvette Cooper welcomed the move, but said ministers should have moved earlier.
She said: “We desperately need them to learn the lesson of the importance of acting quickly on Covid border measures instead of having to face enormous pressure every time to finally act.”
The BBC reported that the British Government’s Scientific Advisory Group on Emergencies (Sage) on Monday decided that pre-departure tests would be “valuable”.
The UK Health and Safety Agency (HSA) said today that a further 26 cases of the Omicron variant have been reported across the UK – 25 of them in the UK.
It takes the total number of confirmed cases of the variant in the UK to 160, including 30 in Scotland.
A risk assessment from the HSA rated the Omicron variant as “red” for the severity of the infection and “amber” for transmission between humans.
It said the variant would likely reduce protection against both natural or vaccine-acquired immunity.
However, it acknowledged that so far there was “insufficient data” to reach firm conclusions and the assessment was presented with “low confidence”.
HSA CEO Dr Jenny Harries said: “We are working as soon as possible to gather more evidence of any impact that the new variant may have on the severity of the disease or the effectiveness of the vaccine.
“Until we have this evidence, we must exercise the highest degree of caution in drawing conclusions about significant risks to people’s health.”
Covid Scotland: 14 deaths in one day while Omicron continues to spread
The Scottish Passenger Agents’ Association (SPAA), which said it represented “really battle-weary” travel agents and companies, described the new tests as a “crushing blow”.
President Joanne Dooey said: “The confidence that returned to international travel will be stamped after this announcement.
“It leaves everyone currently abroad who has a planned return to the UK from 1 p.m.
“We expect travel agents’ phones to ring off the hook at customers who want to cancel or postpone their vacations, business trips and Christmas visits to families abroad.
“The reintroduction of pre-departure testing and the fact that countries can be placed on the ‘red list’ with less than 48 hours’ notice will throw agents back into a chaotic world of client repatriations.
“We ask the Scottish Government that our members need sector-specific support immediately.”
The Airport Operators Association said it would have a “devastating” impact on tourism and aviation.
CEO Karen Dee said: “Tests before departure seem like a big deterrent to travel, and most of the limited residual demand for the reintroduction of self-isolation will now fall away, just as airports were hoping for a small increase over the Christmas holidays.
“Travel and aviation are the only sectors affected by operating restrictions in response to the Omicron variant.”
Rory Boland, which one? Travel editor, said: “Travelers will understand the need for the government to introduce public health measures, but constantly changing rules at the last minute leave passengers to pay the bill.
“In some destinations, it will be difficult for people to secure tests at such short notice.
“For those who test positive abroad, they should first and foremost follow local health guidelines.
“This is likely to require quarantine at your existing hotel or at public housing.
“In some countries, travelers have to pay for quarantine, and some travel insurance providers can help with this.
“Travelers should also check their airline’s flexible booking policy to rearrange flights.”
British Airways chairman and CEO Sean Doyle said: “The general reintroduction of tests to enter the UK, on top of the current regime of isolation and PCR testing on arrival, is completely out of step with the rest of the world, with every other country takes a measured approach based on science.
“Our customers will now be confronted with uncertainty and chaos, and it is once again a devastating blow to everyone working in the travel industry.”