Public Health bosses in the region have released new "urgent advice" which says people who have travelled to the Black Country from the South East or Wales should "assume they have the new variant", even if they have tested negative.
And Christmas plans should be changed to be as short as possible and involve as few people as possible, according to directors of public health in the West Midlands.
The guidance comes 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.
People from Tier 4 areas are now banned from travelling to other parts of the UK such as the West Midlands, which remains in Tier 3.
However analysis of coronavirus swabs shows that the new variant of coronavirus has been in the region since at least early November and was responsible for 11 per cent of cases in the West Midlands in the week starting December 9.
Government guidance says people do not need to self-isolate if they have tested negative but local health leaders are now urging anyone from Tier 4 areas or Wales to avoid contact with others, regardless of any test results.
A statement issued by the West Midlands Local Resilience Forum said: "We are asking anyone who has travelled to the region from any Tier 4 areas or Wales, to assume they have the new variant of Covid and to self-isolate for at least 10 days. This means remaining inside the house where they’re staying for the whole 10 days. 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.
"Although our region is not in Tier 4, rates are increasing and it is highly likely that the new variant is circulating. So please reduce your contact to a minimum, act like people you see have the virus. Remember that people may have the virus and show no symptoms.
"Think ‘should I do this given what we know about the new form of the virus’ not ‘can I do this within the rules’."
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The Local Resilience Forum is made up of local authorities, emergency services and the NHS from the four Black Country boroughs, Birmingham, Coventry and Solihull.
They have issued the advice after the Prime Minister said the new variant was up to 70 per cent more transmissible than the original strain and Health Secretary Matt Hancock admitted it was “out of control” and the Tier 4 lockdown may have to remain in place for months.
The advice continued: "Change your Christmas plans as much as you can. You can only meet your Christmas bubble on the 25th December. Keep your visit as short as possible, with as few people as possible and outdoors if you can.
"If you have Covid symptoms you must get tested through the national booking system. You must self-isolate for 10 days: don’t wait for your test result to start self-isolating. Your close contacts must also self-isolate for 10 days.
"Please only use the national booking system or 119 to book a test if you have symptoms. This means that people who most need a test will be able to get one.
"If you need help with food or money while you self-isolate, please ask for help.
"Look out for other people: pick up the phone, see people on line and check if they are OK. Ask for help if you’re not − it’s OK to feel sad, frustrated or out of hope."
Separate advice has been issued in Staffordshire, which has its own resilience forum, where health bosses have encouraged anyone who has travelled from Tier 4 areas to get tested but have not said they need to self-isolate.
Dr Richard Harling, the director for health and care at Staffordshire County Council and chair of the strategic coordinating group, confirmed there have so far been no confirmed cases of the new Covid variant in Staffordshire.
He said: “There is still a lot we do not know about this new strain, such as whether already having had Covid gives you immunity from it.
“But we do know that it seems to spread very quickly.
“So it is more important than ever that people follow the rules and avoid any unnecessary contacts.
“We are also advising anyone who has returned to Staffordshire or Stoke-on-Trent from London or the South East to arrange a Covid test even if they feel well and have no symptoms either by calling 119 or booking a test online
“It is inevitable cases of the new strain will arrive in our area, but we can all do our bit to slow it down and protect the vulnerable and our services - remember hands, face, space.
“Please take your coronavirus advice from official sources rather than social media.”
Dr Harling added: “We don’t have any confirmed cases of the new strain in Staffordshire, but due to the increased risk of transmission with this new strain, we have to prepare for the likelihood it will travel to Staffordshire if it hasn’t already.
“The only way to prevent its spread is to strictly follow the rules and avoid unnecessary household mixing and travel. Staffordshire remains in a very high alert area and our hospitals will be under immense pressure over the next few months, so we are urging people to stay vigilant, have a smaller Christmas this year and restrict contact with their extended families as much as possible.”
The Staffordshire Resilience Forum consists of the blue-light emergency services, all local NHS organisations, 10 local authorities and local representatives from a number of national agencies.