The British government is banning all non-essential travel outside London and parts of the south-east of England.
The regions are going into Tier 4 public health restrictions to try and stop the spread of the new strain of COVID-19 there.
Last month, four in ten Irish people living in the UK said they planned to come home for Christmas.
But at a press conference this afternoon, Boris Johnson told reporters he's banning travel out of the worst-hit areas.
He also announced that the planned five-day easing of restrictions over the festive period will now be limited to a single day.
In a dramatic move, prompted by fears over a new strain of coronavirus, all those areas currently in Tier 3 in the South East - including London - will move to the new Tier 4 on Sunday.
The Tier 4 measures will see all non-essential shops, gyms, and hairdressers close, with people ordered to stay home apart from limited exceptions such as work - if people cannot work from home - education, childcare and exercise.
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People in Tier 4 areas will no longer be allowed to form "Christmas bubbles" with other households.
Mr Johnson said: "We cannot continue with Christmas as planned."
The action has been prompted by the UK government's fears over a new strain of coronavirus - known as VUI-202012/01 - which is believed to spread more quickly than the original strain and is judged to be the reason for rapidly rising infection rates in the South East.
Mr Johnson said it could be up to "70% more transmissible", and that the situation had given him "no alternative" but to toughen up restrictions.
"Our advisers on New and Emerging Respiratory Virus Threats Advisory Group have spent the last few days analysing this new variant - there's no evidence that it causes more severe illness or higher mortality but it does appear to be passed on significantly more quickly," he said.
Additional reporting by IRN