The UK government declared an "imminent threat" from the new Coronavirus because someone threatened to leave a quarantine centre, the Mirror understands.
A source said the reason is because an unknown person indicated they might "abscond" from the Arrowe Park Hospital, Merseyside.
The new laws could give police or security the power to physically stop someone leaving the quarantine centre. It is understood the person in question is still there.
A source said this very specific issue - and not the coronavirus itself - is what the government deemed an "imminent threat" this morning.
The overall threat to the British public remains 'moderate' and has not changed.
It is not known why the person threatened to leave or what the wider circumstances were.
Health Minister Matt Hancock revealed the emergency powers in an announcement that gives the government extra legal force to fight the spread of the virus.
A statement from the Department of Health said: "In accordance with Regulation 3, the Secretary of State declares that the incidence or transmission of novel Coronavirus constitutes a serious and imminent threat to public health.
"The measures outlined in these regulations are considered as an effective means of delaying or preventing further transmission of the virus."
A source told the Mirror: "Someone was threatening to abscond over the weekend.
"These regulations are to ensure we have the powers to enforce isolation if necessary.
"If they do try to leave, we have the power to stop them."
The source said the move was as much for the person's own safety as the safety of the public. They added: "Overwhelmingly everyone who has been in quarantine has been really good about it. Many of them refused to get on the plane unless they would be quarantined afterwards."
There are now eight confirmed cases of coronavirus in the UK after the number doubled from four this morning.
83 British nationals were flown home from virus epicentre Wuhan for two weeks of quarantine at Arrowe Park on January 31 - meaning they are due to leave this Friday.
They were expected to be joined by other 11 Brits last week when more evacuation flights arrived.
It is thought those who arrived on January 31 signed a contract agreeing to stay in isolation for 14 days.
But after one person said they wanted to leave, officials feared this contract would not give enough legal powers to stop the person exiting the building.
Known as the Health Protection (Coronavirus) Regulations, the law is an emergency power made by the Health Secretary under section 45 of the 1984 Public Health Act.
It will let police wearing protective gear restrain someone if they try to leave quarantine.
According to the 1984 Act, ministers could also force parents to keep a child away from school; ban certain events or gatherings; or put restrictions on the transport of human remains.
On top of all that, the 1984 Act also allows a "special restriction or requirement" to be enforced by magistrates.
However, it's thought the new powers contain no criminal offences and will not criminalise those in quarantine.
It is thought the law came into effect when the Health Secretary signed it this morning, and will lapse if not approved by Parliament within 28 days.
The law identifies Arrowe Park Hospital and Kents Hill Park in Milton Keynes as "isolation" facilities and Wuhan and Hubei province in China as an “infected area”.
It comes after a British man who caught coronavirus in Singapore appears to be linked to at least seven other confirmed cases in England, France and Spain.
Health officials are not confirming a link or giving detail on his relationship to the other people diagnosed with the illness.
But he is reported to be a middle-aged British man and is understood to have been the first UK national to contract the disease.
There have been more than 40,000 cases of the virus globally, mostly in China, while the death toll in China now stands at 908.
Meanwhile, another plane carrying people evacuated from Wuhan landed at RAF Brize Norton on Sunday morning.
Foreign Secretary Dominic Raab said the flight had brought back 105 British nationals and family members, as well as 95 European nationals and family members.
Elsewhere, 60 more people on the quarantined Diamond Princess cruise ship in the port of Yokohama have tested positive for coronavirus, Japan's health minister has said.
There are now 130 confirmed cases on the ship, with officials previously saying 70 people had the virus among the 3,711 passengers and crew.
More than 3,600 people, including 78 British passport holders, are still in a 14-day quarantine on board the ship.
