Video report by ITV News Correspondent Dan Rivers
One of the people diagnosed with coronavirus in the UK attended a conference in central London with more than 200 other delegates, according to organisers.
The person, who has not been identified, attended the UK Bus Summit at the Queen Elizabeth II Centre in Westminster last Thursday.
The conference organiser, Transport Times, sent an email yesterday to all attendees to inform them of the risk, enclosing a letter from Public Health England (PHE).
"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.
It warned attendees to take action and contact the NHS should they develop symptoms such as a fever or a cough.
A copy of the letter from Public Health England sent to attendees of the conference.
Labour MP for Leeds North West, Alex Sobel, said he attended the bus summit and had checked himself in for tests as a precaution.
"I'm absolutely fine, I'm not showing any symptoms... I'm intending to isolate myself until next Wednesday evening which means 14 days will have passed since the UK bus summit," he said.
Labour MP Alex Sobel attended the same conference as one of the confirmed coronavirus patients in the UK.
He said he wasn't properly registered at the event - as he was attending a side event just outside the main conference - and didn't receive a letter from Public Health England right away.
"It really does need a big public health effort and we need many more resources to be turned by the government into information sharing, into ensuring the contagion isn't spread," he said.
The Queen Elizabeth II Centre's website says it hosts up to 400 national and international events a year.
It's located opposite Westminster Abbey and near the Houses of Parliament, and it hosted the Conservative Party's leadership conference last year.
Transport Minister for Roads and Security Baroness Charlotte Vere was in attendance, as well as a number of industry CEOs and directors from across the UK.
Dr Yimmy Chow from PHE said it was unlikely there was "significant" contact between the patient and other delegates, and the letter was a "precautionary" approach.
"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," he said.
Professor Paul Cosford from Public Health England speaks about the ninth person diagnosed with the coronavirus in the UK.
It's understood the person who attended the conference is not linked to the ninth person identified with the virus in the UK - a woman from China who took an Uber to A&E after she developed symptoms.
The unnamed woman is being treated at a specialist NHS centre at Guy's and St Thomas, and officials believe she contracted the virus in China before arriving in 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.