The new strain of coronavirus is probably already in Northern Ireland and the Republic, Arlene Foster has warned.
Ms Foster said ministers were very concerned about the highly infectious mutation and tests were ongoing.
She told the BBC's Good Morning Ulster programme: "It is probable that it is here and likely it is in the Republic too."
The Stormont executive has slashed the five-day holiday relaxation of restrictions, allowing three households to gather, to just Christmas Day.
Flexibility will be afforded to people working on December 25 to enable them to join others on another day over the festive break.
The decision was made by ministers during an emergency virtual Executive meeting late on Sunday night.
The DUP leader urged people to follow the new rules.
She deeply regretted that official guidance on the holiday season had changed.
Ministers also debated imposing a temporary ban on travel from Britain to Northern Ireland in response to the new variant of Covid-19 that has taken hold in London and Kent.
No decision was taken but ministers are expected to return to the issue today.
A swathe of European countries including the Republic have imposed bans on travel from the UK and this has halted some freight movements.
Stormont Executive ministers debated amending the 1967 Public Health Act to introduce short-term travel restrictions.
Sinn Féin supported the move, however practical issues were raised around how quickly such a ban could be introduced, given the time it would likely take for the Department of Health to draw up regulations to bring it into effect.
The DUP position was that UK internal travel is already banned from Tier 4 areas in England, so there is no need for added Stormont legislation to enforce that.
The party believes the responsibility for stopping flights and ferries within the UK is an issue for the British government and has also raised concern that Stormont could leave itself open to compensation liabilities if it was to legislate on the issue.
The Department of Health is to seek further legal advice.
Health Minister Robin Swann is to consult with the attorney general about the legality of a ban on travel from Britain.
He is to give a statement in the assembly at midday today.
Mobility data suggests the stay at home guidance during the north's recent two-week circuit-breaker was not adhered to properly.
The health minister wants to put the stay at home messaging into the coronavirus regulations, to make it legally enforceable, subject to exemptions such as travel for work.
He is also seeking a high-profile policing operation.
