Anyone who travelled from a Tier 4 area or Wales is being urged by health chiefs to self isolate, turn away Christmas Day visitors and "assume" they have the new Covid-19 variant.
The "urgent advice" from public health directors in the West Midlands was issued through the region's strategic co-ordinating group (SCG) on Monday, after London and much of the South East entered Tier 4 at the weekend.
In Wales, tough new measures also came into force on Sunday.
The English region's move came after widely publicised scenes of people packing railway stations in London hours before Tier 4 controls came into force at midnight on Sunday, prompting fears the more infectious new variant could be spread across the country by the travellers.
In a statement issued on behalf of all the region's health chiefs, the SCG urged people to "self-isolate for at least 10 days".
"This means remaining inside the house where they're staying for the whole 10 days," they said.
"Other people who live in the house do not need to self-isolate unless they get symptoms, but no visitors should be allowed in that house at all, even on Christmas Day.
"Even if you have a negative test, you should continue to self-isolate."
The statement sent by the West Midlands Local Resilience Forum on behalf of the SCG added that although the West Midlands is not in Tier 4, "it is highly likely that the new variant is circulating" in the region.
Health chiefs also urged people to remember that those infected may show no symptoms and, regarding their own plans, be thinking "'should I do this given what we know about the new form of the virus' not 'can I do this within the rules"'.
The SCG statement urged people to "change your Christmas plans as much as you can", and to keep visits on the day "as short as possible, with as few people as possible and outdoors if you can".
The impact of the new variant had an immediate effect at the weekend, with Prime Minister Boris Johnson also scrapping a planned five-day easing of restrictions, instead limiting the relaxation to just December 25.
Crisis talks are also being held to prevent the UK being cut off from the continent over fears about the spread of the new coronavirus variant, as countries including the Netherlands, France, Germany moved to curb travel.
Canada, India and Turkey have also imposed restrictions on UK travel.
