Northern Ireland has confirmed a case of the new Covid-19 variant that was first detected in Great Britain.
One positive result has been reported after genome analysis was conducted on a small number of suspected cases.
It is understood health officials believe the strain has been in circulation at a low level in Northern Ireland for several weeks.
The development came as Northern Ireland prepares to enter a new lockdown on Boxing Day.
Health Minister Robin Swann said: “This is sadly the confirmation we had been expecting.
“As I have stated from the outset of this pandemic, we have to avoid both panic and complacency.
“We all have to redouble our efforts to stop the virus spreading. We know how to do this – cut down our contacts with others, ensure strict social distancing, wash our hands regularly and thoroughly, and wear a face covering.”
Chief medical officer Dr Michael McBride added: “While virus mutation is not uncommon, the potential of this new strain to spread rapidly is cause for serious concern.
“I would advise the public to act on the assumption that it is already well established in Northern Ireland and that the person they pass in the street or stand next to in a queue may have it.”
Under new Stormont guidance, issued in response to the emergence of the variant in England and Wales, anyone returning to Northern Ireland for the festive season should be self-isolating on Christmas Day.
The advice, published by the executive earlier on Wednesday, comes after the executive decided to recommend against any non-essential travel between Northern Ireland and both Great Britain and the Irish Republic.
The executive also agreed to advise anyone arriving in Northern Ireland, who is staying for more than 24 hours, to isolate for 10 days. It applies to anyone who has arrived since December 22.
The guidance, published by the devolved administration, confirms that people who regularly cross the Irish border for essential reasons, such as going to work, do not need to self-isolate.
On Wednesday, new regulations also came into force that gave legal footing to the executive’s decision to reduce the five-day period of Christmas relaxations on household gathering to just one day between December 23-27.
Covid rates in the region remain high, with another 21 deaths and 787 new cases confirmed on Wednesday.
The first week of Northern Ireland’s new lockdown will be the toughest so far imposed in the region.
A stay-at-home curfew will be in place from 8pm to 6am for that week.
“It is essential that the maximum benefit is secured from the lockdown that starts on Boxing Day,” said Mr Swann.
“That’s how we keep ourselves safe, protect the vulnerable in our society and protect our health service.
“This lockdown will only work if we all fully play our part – strictly follow the Covid regulations and public health advice and be ultra-careful in everything we do.
“I would urge everyone to review their plans for Christmas and to err on the side of caution. Just because you can do something, it doesn’t mean you have to.”
The guidance on travel was introduced after a Sinn Fein bid in the executive to introduce a legal ban on travel between Northern Ireland and Great Britain was voted down by other ministers.