Eight planes were reportedly placed on lockdown over coronavirus fears.
One jet understood to be a United Airlines plane - flight 901- from San Francisco, was held on the runway this morning as passengers were told to remain in their seats.
A passenger on board took to Twitter to say travellers had been asked to fill in a form and the jet was then reportedly cleaned by experts in hazmat suits.
Another said, Andy West, from Henley-on-Thames, told the Mail the passenger who told crew they felt unwell was taken to the back of the plane while health officials were called.
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."
When asked, Heathrow did not confirm or deny eight planes were being held.
Nine people in the UK have been confirmed as having the virus.
Yesterday it was revealed a woman infected with coronavirus attended a bus conference in London that had more than 250 delegates.
Among those speaking in the QEII centre in Westminster on February 6 was Boris Johnson's buses minister Baroness Vere of Norbiton.
Less than three days after the conference the woman, a Chinese national, fell ill and took an Uber to University Hospital Lewisham.
She is now in isolation at St Thomas' hospital near London Bridge.
Her presence in central London at a packed conference will spark fears among the hundreds of people there from the transport industry amid growing public anxiety about the spread of the deadly bug, which has infected 64,441 people worldwide and killed 1,383.
It is not known how the woman travelled to the conference centre, which is roughly 100 yards from Westminster Underground station.
Experts have warned Covid-19 could spread rapidly on the London Underground system.
The summit's speakers included Baroness Vere, who posed with an all-electric bus outside the centre, as well as David Brown, Chief Executive of bus company Go-Ahead, Nottingham South MP Lilian Greenwood and Gareth Powell, head of Transport for London's bus network.
All attendees at the UK Bus Summit have been emailed warning them that someone at the summit had coronavirus, according to the Financial Times.
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