One of the people diagnosed with coronavirus in the UK attended a central London conference with more than 200 delegates.
The person, who has not been named, attended the UK Bus Summit at the QEII Centre in Westminster on February 6.
Transport Times, the conference organiser, emailed attendees on Thursday afternoon informing them that a person confirmed to be suffering from coronavirus illness - also known as Covid-19 - had been at the event.
Dr Yimmy Chow, consultant in health protection at Public Health England (PHE), said: "One of our main priorities has been to identify any people who we think have been in close contact with confirmed cases of Covid-19 to provide public health advice, as they may be at slightly increased risk of catching the virus.
"While the degree of contact conference delegates may have had with the case is unlikely to have been significant, we have taken a precautionary approach and informed them of the situation."
The PA news agency understands that the case is not linked to the ninth patient with coronavirus in the UK - a woman from China.
The Transport Times email enclosed a letter from PHE saying delegates should take no action if they were well but to stay indoors, avoid contact with others and call NHS 111 if they developed symptoms such as a fever or cough.
"While the degree of contact you may have had with the case at the summit is unlikely to have been significant, we are taking a precautionary approach and informing you," the letter said, as reported by the Financial Times.
The bus conference listed about 250 delegates from the nationwide bus and transport industry.
Among the speakers listed were buses minister, Baroness Vere and the former parliamentary under secretary of state transport, Nusrat Ghani.
Labour MP for Nottingham South, Lilian Greenwood, who also spoke at the summit, tweeted that she had received PHE's letter.
She added: "I'm feeling completely well but to be extra-cautious I'm cancelling my public engagements until 20th February. Sorry for any inconvenience."
It comes as United Airlines confirmed it was offering assistance at Heathrow Airport on Friday after a passenger fell ill.
Reports on social media suggested the person thought they may have coronavirus.
United Airlines said in a statement: "Our team at London Heathrow Airport is providing assistance related to United flight 901 (San Francisco-London Heathrow) today, following reports of an individual becoming unwell onboard.
"The safety of our customers and employees is our highest priority and we continue to work closely with local authorities."
Heathrow Airport declined to comment on reports other planes were also held due to coronavirus fears.
Meanwhile, a Channel 4 employee is among those being tested for coronavirus after feeling unwell following a trip to Asia.
A Channel 4 spokesperson said: "On Thursday a member of staff at Channel 4 who had travelled to Asia within the last month felt unwell.
"As a precautionary measure they decided to seek medical advice and, in line with the latest public health advice regarding the coronavirus, they were taken to hospital for a precautionary test. We have informed our staff of this and continue to follow all the latest public health guidance."
NHS chief executive Sir Simon Stevens said on Thursday that many more people in the UK may need to self-isolate to contain the illness, which has been officially named Covid-19.
Also on Thursday, more than 80 people quarantined at Arrowe Park Hospital on the Wirral were allowed to leave following 14 days in isolation, with Health Secretary Matt Hancock confirming they pose "no risk to the public".
Suspected cases of coronavirus are still being tested, including one mother who told the Sun that her baby son had come into contact with an infected doctor while being treated for a leg injury at Worthing Hospital.
Stephanie Adlan, 28, said she was "terrified" for eight-month-old James and has self-isolated with her family but criticised medics at the hospital for a lack of information.
She told the paper: "I've had nothing from the hospital. Not a 'How are you?' or 'What's going on?' 'How's the baby?' We've just been told to stay indoors and call 111 if our symptoms get worse."
It comes as it emerged that the woman who is the UK's ninth case of coronavirus took an Uber to A&E after she developed symptoms.
The unnamed woman, who is being treated at a specialist NHS centre at Guy's and St Thomas' in central London, contracted the virus in China before flying to the UK.
Two staff from Lewisham Hospital in south London are now in isolation at home after coming into contact with the woman.
In China, the number of deaths from coronavirus has reached 1,380, with more than 63,000 recorded infections, in figures announced early on Friday morning.
A total of 44 more people on quarantined cruise ship the Diamond Princess in Japan have tested positive for Covid-19. Two of those are Britons, taking the number of Britons on the ship diagnosed with coronavirus to three.