One of the people diagnosed with coronavirus in the UK attended a central London conference with more than 200 delegates, it has been reported.
The patient, 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 on the instruction of Public Health England (PHE), informing them that a person confirmed to have the virus had been at the event, according to the Financial Times.
The 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.
The bus conference listed about 250 delegates from the nationwide bus and transport industry.
It comes after a coronavirus expert warned that the spread of misinformation and conspiracy theories could lead to more cases in the UK.
Professor Paul Hunter, of the University of East Anglia's (UEA) Norwich Medical School, said fake news leads to bad advice and people taking "greater risks" during health crises.
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".
Prof Hunter, who has previously run tests on the impact of fake news on disease outbreaks with colleague Julii Brainard, said speculation was already rife online about the origin of the virus and how it can be spread.
He said: "Misinformation means that bad advice can circulate very quickly - and it can change human behaviour to take greater risks."
He added: "Examples of risky behaviour during infectious disease outbreaks include not washing hands, sharing food with ill people, not disinfecting potentially contaminated surfaces, and failing to self-isolate.
"Worryingly, people are more likely to share bad advice on social media, than good advice from trusted sources such as the NHS, Public Health England or the World Health Organisation."
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.
The Department of Health said on Thursday that 2,521 people in the UK have been tested, of whom 2,512 were confirmed negative and nine positive.
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.