Here are the main points from today's Scottish government briefing:

• The planned five-day relaxation of Covid restrictions over the festive period has been cut back to just Christmas Day for the whole of Scotland.
• Most of Scotland faces a return of the toughest level four restrictions from Boxing Day. They will last for three weeks and apply to all areas except the islands which will be placed in level three.
• Cross-border travel has been banned and it will be against the law to travel outside local council areas - except for essential reasons.
• Most school children in Scotland will be taught online until at least 18 January – with a phased return to schooling from 11 January – except for the children of key workers.

That's the end of our live page coverage on a day of fast-paced developments. Stay safe.

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Police Scotland has said its officers would not be routinely stopping people to enforce the new travel restrictions and would use common sense and discretion.

Assistant Chief Constable Alan Speirs said the majority of the public had been complying with existing rules.

He said: “Our officers will continue to engage with the public, explain the legislation and guidance, and encourage compliance. We will use enforcement as a last resort where there is a clear breach of the legislation.

"We have been very clear that we will not be routinely stopping vehicles or setting up road blocks. However, officers may in the course of their duties come across people who are travelling from one local authority area to another."

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Ms Sturgeon is asked how the new strain appears to be spreading in hospitals and care homes.

She says health experts have been puzzling over why there are currently fewer care homes with outbreaks but each care home has been seeing more people infected than was previously the case.

"It may be that it is not this faster transmitting virus that is responsible but this raises the question in our minds 'is that the case?', Ms Sturgeon says.

On hospitals, she says there has been a rapidly deteriorating situation in the Borders that seems to have started within Melrose hospital.

Again, it raises the question of whether the new strain is responsible, she says.

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Ms Sturgeon is asked how willing people will be in adhering to the guidelines.

The first minister says the travel restrictions will be the law. "They are not guidelines, they are law," she says.

"The law will say that throughout this period it is not permitted to travel from Scotland to the rest of the UK or vice versa," she says.

Similarly restrictions on travel outside your local council area are in place for those in level three.

From Saturday, almost all the country will be in level four and there will be no travel outside local council areas unless essential, she says.

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The Scottish Liberal Democrat leader has said schools that are set to open this week should close immediately.

Willie Rennie said: “When the science speaks in such stark terms, the leadership of the country must listen and act.

"The new strain of the virus is already here and pupils and teachers who are infected in school next week could pass it onto vulnerable relatives on Christmas Day.

BBC Scotland's Glenn Campbell says the first minister was never keen on easing the restrictions over the Christmas period.

"With hindsight do you wish you had not bothered?" he asks.

Ms Sturgeon says she always "agonised" over the decision.

She says she also wanted people to only use the relaxed rules if it was essential.

It was in recognition that some people would find it difficult to leave loved ones alone over Christmas, she says.

If it was not for what she had been told today about the new strain she would not be changing the rules, Ms Sturgeon adds.

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Douglas Ross, leader of the Scottish Conservative Party, said families "would be devastated" at the new measures so close to Christmas.

However, he added: "Given the rapidly increasing spread of the virus across the UK, and until our scientists know more about the transmission of this new variant, it is understandable why these restrictions are necessary at this time."

The first minister concluded her briefing by saying there was light at the end of the tunnel.

The vaccination programme is progressing at pace, she said.

We can't see the light at the moment but it has not gone out, she says.

"We do have brighter times ahead but to get you to them I have to ask you for more sacrifices," she says

Ms Sturgeon says: "Please believe me when I tell you that I would not be standing here on the Saturday before Christmas if I did not think this was necessary."

She says: "I have spent many anxious and worried days over the past 10 months but I have rarely felt more worried as I have today."

The first minister says she wants to do everything she can to reduce the possibility of the new strain overwhelming us.

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Level four restrictions will apply to all of mainland Scotland for three weeks from one minute after midnight on Boxing Day morning, Ms Sturgeon says.

That means all non-essential shops, pubs, cafes and restaurants will close

Orkney, Shetland, the Western Isles and the other islands with reduced restrictions will not be in level four. They will be in level three with strict restrictions on who can travel there.

The level four restrictions will be reviewed after two weeks.

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The education secretay has said the decision to move to online learning for most pupils at the start of term was a diffcult one.

John Swinney said schools that planned to open this week would still open. Schools would then return from 5 January for staff, vulnerable pupils and children of key workers.

All other pupils would start remote learning on 11 Jan with the aim being a full return on 18 January. This will be subject to review.

People with caring responsibilities can still visit other people to do that, Ms Sturgeon says.

If care for vulnerable people is essential it can be done on any day.

"Standing here saying this actually makes me want to cry," says the first minister.

"I'm sure listening to it will make many of you want to cry.

"I know how harsh it sounds. I know how unfair it is but this virus is unfair.

"It does not care about Christmas. It does not care about anything other than spreading itself as far and wide as possible.

"It has just become, unfortunately, a lot better at doing that."

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Any schools which were due to be open next week, should be closed, the Scottish Greens have insisted.

The party has backed the Scottish government's move to tighten the Christmas rules.

Co-leader Patrick Harvie said: “It’s clear that the increase in cases linked to a new strain are very concerning indeed.

"For weeks Scottish Greens have been raising the concerns of public health experts that the planned relaxation of restrictions would lead to a significant rise in infections of COVID-19, so in the face of an even more transmissible strain this extra caution from the First Minister is necessary."

He added that the staggered school return was a sensible approach.

Mr Harvie added: “Many people will be baffled that the government is still insisting on opening schools for two days this week, and I would urge local authorities to close early to keep staff, pupils and communities safe."

Ms Sturgeon says travel within Scotland will be allowed on Christmas Day but should only be used if really necessary.

She also says household limits will still apply on Christmas Day - a maximum of eight people from three households.

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The first minister says the five-day relaxation of the rules from 23 to 27 December had been planned to recognise the loneliness and the difficulty we all feel in leaving loved ones alone over Christmas.

She says that given the concerns over the new strain it will now only allow indoor mixing in a "bubble" for one day only - Christmas Day.

"We will allow Christmas Day to go ahead but only use this flexibility if you really truly need to," she says.

"If you had people travelling to join you from elsewhere in the UK that will no longer be permitted."

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Richard Leonard, the leader of Labour north of the border, says the Scottish government needs to make decisions based on public health advice and "solid evidence".

Ms Sturgeon says she is taking action to prevent things getting worse, potentially very quickly.

She says failing to act quickly is always a mistake.

Ms Sturgeon says there will be a strict travel ban between Scotland and the rest of the UK, right throughout the Christmas period.

"That means people from Scotland not visiting other parts of the UK and vice versa," she says.

Cross-border travel for all but the most essential purposes is not permitted, she says.

The position on international travel is being considered.