Health officials are urgently trying to trace those who came into contact with the first two cases of coronavirus recorded in the UK.
Public Health England confirmed two people from the same family were being treated for the virus in Newcastle.
The update came just hours before the arrival of a plane from Wuhan in China carrying British nationals from the epicentre of the disease back to Britain.
It is unclear where the two patients are from, but it is understood they are not from the Wirral area, where a special facility has been set up to quarantine those returning from Wuhan.
An outbreak investigation team has been formed to trace anyone who has been in contact with the pair to prevent the illness spreading.
The pair had been staying at the Staycity apartment-hotel in York when they became unwell. It is understood that they travelled to the UK from China recently, they are now undergoing treatment at the Newcastle Royal Victoria Infirmary.
The firm has said the apartment involved has been thoroughly disinfected and Public Health England has been providing support.
An evacuation flight carrying British nationals from Wuhan lands in Oxfordshire:
Eighty-three Britons and 27 foreign nationals landed at RAF Brize Norton at around 1.30pm on Friday on an evacuation flight from Wuhan.
The passengers are expected to make a three-and-a-half hour journey on coaches to Arrowe Park Hospital in the Wirral, where they will remain in quarantine for 14 days.
In a statement, Professor Chris Whitty, said: "We can confirm that two patients in England, who are members of the same family, have tested positive for coronavirus.
"The patients are receiving specialist NHS care, and we are using tried and tested infection control procedures to prevent further spread of the virus.
"The NHS is extremely well-prepared and used to managing infections and we are already working rapidly to identify any contacts the patients had, to prevent further spread.
Briton Matt Raw records a message as he heads from RAF Brize Norton to the quarantine facility on the Wirral
Prof Whitty said the virus remains "moderately transmissible" and has a mortality rate of around two per cent currently.
This is much lower than other disease outbreaks such as Ebola, which had a 70% mortality rate, and SARS, which had a 10% mortality rate, he said.
He added: "Nevertheless, if we got very large numbers that is obviously a concern and this would be something we would have to take very seriously."
Dr Jenny Harries, deputy chief medical officer for England, said: "Everybody who has got on the plane is a well passenger.
If any of those passengers do show symptoms there are set procedures to isolate them during any process and remove them in any part of the journey."
She said all passengers had had thermographic screening before boarding the plane and were subject to ongoing questioning and continuous risk assessment during the journey.
Public Health England revealed at least 1,466 passengers and 95 staff arrived in the UK on direct flights from Wuhan between January 10 and 24 - prior to a transport ban being implemented in Wuhan.
On Wednesday British Airways suspended all flights to and from the country after the Foreign Office warned against "all but essential travel".
New figures suggest up to 35,000 passengers could be affected by the cancellation of flights to mainland China by UK airlines due to the coronavirus outbreak.
Prof Whitty urged people who had come from anywhere in China and show symptoms associated with the coronavirus should "self-isolate" for 14 days.
The UN health agency defines an international emergency as an “extraordinary event” that constitutes a risk to other countries and requires a coordinated international response.
China first informed WHO about cases of the new virus in late December.
Experts say there is significant evidence the virus is spreading among people in China and have noted with concern instances in other countries - including the United States, France, Japan, Germany, Canada, South Korea and Vietnam - where there have also been isolated cases of human-to-human transmission.
Speaking to reporters in Geneva on Thursday, WHO Director-General Tedros Adhanom Ghebreyesus noted the worrisome spread of the virus between people outside China.
“The main reason for this declaration is not because of what is happening in China but because of what is happening in other countries,” he said.
The new virus has now infected more people in China than were sickened there during the 2002-2003 outbreak of SARS, or severe acute respiratory syndrome, a cousin of the new virus.
At least 213 people in China have died from the disease and there has been almost 10,000 recorded cases.
There has been nearly 100 recorded cases of the virus in 18 countries. No deaths have been reported outside China. Those who have died in Wuhan appear to have had pre-existing health conditions.