The World Health Organization (WHO) has said it is in "close contact" with UK officials over the emergence of a new variant of Covid-19.
The new variant is spreading more rapidly than the original version, but is not believed to be more deadly.
Large parts of south-east England, including London, are now under a new, stricter level of restrictions in a bid to curb the rapidly spreading virus.
The Netherlands said it was banning UK flights because of the new variant.
The ban comes into force on Sunday and will remain in place until 1 January at the latest, the Dutch government said.
The move comes after sampling of a case in the Netherlands earlier this month revealed the same strain of coronavirus as that found in the UK.
Pending "greater clarity" on the situation in the UK, it said "the risk of the new virus strain being introduced to the Netherlands should be minimised as much as possible".
The Dutch government also said it would work with other European Union member states in the coming days to "explore the scope for further limiting the risk of the new strain of the virus being brought over from the UK".
What do we know about the new variant?
The WHO tweeted that it was in contact with UK officials over the new variant.
It said the UK was sharing information from ongoing studies into the mutation, and that the WHO would update member states and the public "as we learn more about the characteristics of this virus variant [and] any implications".
Although there is "considerable uncertainty", British Prime Minister Boris Johnson said the new variant may be up to 70% more transmissible than the old one.
But officials say there is no current evidence to suggest the new variant causes a higher mortality rate or that is affected any differently by vaccines and treatments.
"I think this is a situation which is going to make things a lot worse, but there are some really optimistic things if you look once we get the vaccine out, assuming the vaccine works against this, which at the moment is the working assumption," said England's chief medical officer, Prof Chris Whitty.
What else is happening in Europe?
In the UK, the planned relaxation of Covid rules for Christmas has been scrapped for large parts of south-east England and cut to just Christmas Day for the rest of England, Scotland and Wales.
Italy has ordered a nationwide lockdown over much of the Christmas and New Year period. The country will be under "red-zone" restrictions over the public holidays, with non-essential shops, restaurants and bars closed, and Italians only allowed to travel for limited reasons.
The Netherlands and Germany have imposed lockdowns until January. In Germany, Christmas will see a slight easing, with one household allowed to host up to four close family members.
Austria is set to enter its third lockdown after Christmas. From 26 December, non-essential shops will be shut and movement outside homes restricted.
Sweden has recommended wearing face masks on public transport during the rush hour, reversing its earlier guidance.
France's President Emmanuel Macron is in a "stable" condition after testing positive for coronavirus, his office said on Saturday. He is still experiencing symptoms, such as coughing and fatigue, but they are not preventing him from working, it said.
Slovakia's Prime Minister Igor Matovic, who attended an EU summit with Mr Macron last week, said he had tested positive for coronavirus on Friday.
Several other European leaders who were also at the summit said they would self-isolate.