England's Chief Medical Officer has said the UK has informed the World Health Organization that the new variant coronavirus can spread more rapidly.
Professor Chris Whitty’s comments came as British Prime Minister Boris Johnson holds emergency talks with cabinet ministers to discuss tougher new restrictions.
There are reports Mr Johnson could order new travel restrictions for south-east England in an attempt to stop to new strain spreading to the rest of the country.
Prof Whitty said: "As announced on Monday, the UK has identified a new variant of Covid-19 through Public Health England's genomic surveillance.
"As a result of the rapid spread of the new variant, preliminary modelling data and rapidly rising incidence rates in the South East, the New and Emerging Respiratory Virus Threats Advisory Group (NERVTAG) now consider that the new strain can spread more quickly.
"We have alerted the World Health Organization and are continuing to analyse the available data to improve our understanding.
"There is no current evidence to suggest the new strain causes a higher mortality rate or that it affects vaccines and treatments, although urgent work is under way to confirm this.
"Given this latest development, it is now more vital than ever that the public continue to take action in their area to reduce transmission."
Northern Ireland's First Minister Arlene Foster described the new strain as "very concerning".
She was speaking after taking part in a phone call with British minister Michael Gove and the First Ministers of Scotland and Wales.
There have been 17 further coronavirus-related deaths in Northern Ireland, with 13 occurring during the last 24 hours. It takes the official death toll in the region to 1,183.
640 new Covid-19 cases have also been recorded, taking the cumulative total to 61,437.
There are 427 coronavirus patients in hospital, with 30 in intensive care, 23 of whom are on ventilators.
Just finished a call with the other UK First Ministers & CDL Michael Gove about how the UK handles this new strain of covid prevalent in SE England.
Important we all follow the regulations so we keep virus under control in Northern Ireland.
— Arlene Foster #We'llMeetAgain (@DUPleader) December 19, 2020
Scotland's First Minister Nicola Sturgeon said "preventative action may be necessary" to halt the spread of the variant strain.
Former health secretary Jeremy Hunt warned the situation was on a "knife-edge" with infections rising and hospital admissions on the increase.
Mr Johnson held an unscheduled meeting yesterday evening to discuss the latest evidence about the mutant strain amid "growing concern" in Downing Street.
Earlier, he refused to rule out the prospect of a third national lockdown for England if infection rates continue to grow – a move he has previously been deeply reluctant to contemplate.
Wales and Northern Ireland have already announced fresh lockdowns once Christmas is over, while the Scottish Government has said the option "remains on the table".
Much of the latest focus is on the south and east of England where the new variant is thought to be most prevalent.