Health officials are trying to discover how a man in England caught coronavirus, after he became the first person to infected within the UK.
The man, from Surrey, had not been abroad recently – unlike the 19 others who have tested positive in the UK.
Health minister Helen Whately told BBC Newsnight officials are trying to trace people he was in contact with.
Meanwhile, the government is expected to publish emergency legislation next week, if the virus spreads.
The package of measures would be designed to help the public sector – such as the NHS and schools – cope with the outbreak, if there was a serious rise in cases.
There are few details about what the measures could involve – but school class sizes in England is one area which would be addressed. They would be permitted to rise above statutory limits, in the event of widespread teacher absences.
On Friday, a British man became the first UK citizen to die from coronavirus after being infected on the Diamond Princess cruise ship quarantined in Japan.
Prime Minister Boris Johnson, who said the virus outbreak was now the government’s top priority, will chair a meeting of the government’s Cobra emergency committee on Monday.
He had faced criticism from Labour, who said he needed to “get a grip” on the situation and questioned why he was waiting until next week to hold the meeting.
Nearly 9,000 people in the UK have so far been tested for coronavirus.
On Friday, the number of people who have tested positive in the UK reached 20, after the man in Surrey was diagnosed.
He was the first to catch it in the UK – although it is unclear whether this was directly or indirectly from someone who recently returned from abroad.
He has been transferred to a specialist NHS infection centre at Guy’s and St Thomas’ Hospital in central London. A clinic in Surrey – Haslemere Health Centre – has been closed for “deep cleaning”.
Who did this man catch the virus from?
This is the urgent question that needs answering about the 20th case in the UK.
So far, no connection with anyone who has travelled to an affected country has been discovered.
Until we know the answer it is difficult to know how big a development this is.
This could be an “outbreak of two” – with just one other, still to be identified, person that caught coronavirus abroad.
Or is this the first case to be detected from a much larger outbreak? We know this can happen, Italian scientists believe the virus was circulating there unnoticed for weeks.
For now, we simply do not know, but this is a scenario officials have been preparing for.
Ms Whately said: “At the moment the work is going on to make sure that we have traced the contacts that that individual has. That work is happening at the moment.”
She added: “We are well-prepared but we do have to recognise that it is likely we will see more cases within the UK.
“There will be a plan published next week for the public, giving more detail about the government’s preparedness.”
She said face masks were not recommended for the public, but that “everybody has a part to play to help us be prepared as a country”.
People should wash their hands thoroughly. she added – echoing the advice given by the prime minister on Friday – and carry a tissue to sneeze or cough into, she said.
Of the most recent case, the Department of Health and Social Care said the virus was passed on in the UK but the original source was “unclear” and there was no “immediately identifiable link” to overseas travel.
Prof Jonathan Ball, from the University of Nottingham, said the Surrey case marks a “new chapter for the UK” and that it is “crucial” to understand the infection’s origin.
“This was always a concern – this is a virus that frequently causes symptoms very similar to mild flu or a common cold, and it’s easily transmitted from person to person. This means it can easily go under the radar,” the virology expert added.
It comes as a hospital in Edinburgh is carrying out “drive-through” testing for coronavirus, where patients do not need to leave their cars when they arrive for appointments.
What do I need to know about the coronavirus?
- WHAT ARE THE SYMPTOMS? A simple guide
- WAYS TO PREVENT CATCHING IT: How to wash your hands
- WHERE ARE WE WITH A VACCINE? Progress so far
- A VISUAL GUIDE TO THE OUTBREAK: Virus maps and charts
- WHAT DOES IT MEAN FOR MY HOLIDAY? Your rights as a traveller
Three other cases of the virus were confirmed in the UK on Friday, including the first one in Wales and two in England.
The two new patients in England contracted the virus while in Iran, the Department of Health and Social Care said.
Public Health Wales said it was working to identify close contacts of the Welsh patient, who is believed to be from the Swansea area and was infected in northern Italy before returning to the UK.
Northern Ireland confirmed its first case on Thursday. Authorities said they had contacted passengers who sat near the woman on a flight from northern Italy to Dublin.
The World Health Organization has raised its global risk assessment of the virus to its top level, “very high”.
Globally, more than 80,000 people have been infected. About 2,800 have died – the vast majority in China’s Hubei province.