Since the coronavirus pandemic began, the government has imposed its first outright travel ban on arrivals.
The Department for Transport ( DfT) announced in the early hours of Saturday that foreign visitors from Denmark would be denied entry.
The unprecedented move is due to severe concerns about coronavirus mutations spreading from mink to human beings.
All 17 million mink that are currently being bred on Danish farms are to be culled.
The move followed 24 hours after the quarantine for arrivals from Denmark was suddenly imposed by the DfT.
The order to remove the quarantine exemption from Denmark took effect at 4 a.m. on Friday, giving British travelers no chance to return without the need for quarantine.
The presence of the UK Border Force has been stepped up "to ensure that those coming to the UK from Denmark comply with the new restrictions," said the DfT.
Only British nationals, visa holders, and permanent residents will be allowed in from Denmark until further notice. Anyone who has been in Danish territory for the past two weeks, however, has been subject to the most severe quarantine measures imposed since March.
None of the usual 14-day self-isolation exceptions will be applicable, and all members of the household of the arriving traveler must also be in quarantine.
"The statement read:" By bringing in a number of measures, the government has responded urgently to recent developments.
The Home Office is implementing immigration powers, meaning that visitors coming from Denmark to the UK will not be allowed to enter the UK. This excludes cargo and haulers.
"All non-British nationals or residents who have been in or transiting through Denmark in the last 14 days shall, upon arrival, be refused entry by the Border Force."
"The decision to act quickly follows the release of additional information from health authorities in Denmark reporting widespread coronavirus outbreaks in mink farms, with the spread of a variant strain of the virus to certain local communities."
The government is examining the Passenger Locator Forms filed by arrivals to identify those who have been on Danish soil in the past two weeks, as well as new arrivals from Denmark.
"Anyone who has returned from Denmark in the last two weeks will be contacted to ensure that the self-isolation required to ensure that the virus does not spread throughout the UK is complete," the DfT said.
They will be asked to quarantine along with all other members of their households until 14 days after they were last in Denmark.
The statement added: "The Government of the United Kingdom works closely with international partners to understand the virus changes reported in Denmark and we are conducting a further research program here in the United Kingdom to inform our risk assessments."
Passengers booked on British Airways flight 811 from Copenhagen, which touched down at London Heathrow shortly before 8 a.m. on Saturday, were the first travelers to be affected by the Danish ban.
The second and third BA arrivals from the Danish capital are due at 11:40 a.m. and 7:15 p.m. at Heathrow, as well as the 4 p.m. SAS flight.
In addition, Ryanair has flights from Copenhagen on Saturday to Manchester at 5:25 pm and Stansted at 10:45 pm.
Loganair was the first airline to announce the suspension of flights. The Scottish airline tweeted: "Loganair had to suspend its service between Aberdeen and Esbjerg from 9 to 22 November because of travel restrictions imposed by the government this morning with regard to Denmark."
"Customers will be contacted by email because of travel over this period and offered either a refund or the opportunity to move their booking to another date."
After a week, the travel ban and expanded requirements for self-isolation will be reviewed.
Separately, from 4 am on Saturday, Denmark's neighboring countries, Germany and Sweden, were placed on the UK's no-go list.
