The novel coronavirus can remain airborne in enclosed spaces for at least 16 hours after patients breathe it out.
Prof Wendy Barclay of Imperial College London said there was mounting evidence the virus could be spread in an airborne way too.
In an interview to the BBC Barclay said: “We do know that… the virus that causes Covid can remain viable, remain infectious in these very small droplets.”
“So that raises the possibility and indeed the likelihood that Covid can be transmitted through these small particles that can trace through the air.”
“Laboratory studies where the virus has been purposefully put into the air tell us that the virus can remain there for more than an hour in its infectious form.”
Until now governments have focused on the dangers of touching surfaces that are contaminated with virus.
It comes after a World Health Organisation report said so-called “aerosol” transmission “cannot be ruled out”.
In a major shift, the WHO said “much more research is needed”, but that droplets “could explain human-to-human transmission” in some cases.
World Health Organisation in a report said: “Short-range aerosol transmission, particularly in specific indoor locations, such as crowded and inadequately ventilated spaces over a prolonged period of time with infected persons cannot be ruled out.”
Experiments using high-powered jets had found Covid-19 remained airborne for three hours in one study and 16 hours in another.
However, it added: “These findings were from experimentally induced aerosols that do not reflect normal human cough conditions.”
Prof Barclay said people could slow the spread by being in a well-ventilated room and socially distancing.
She added it is “quite likely” that a face covering will remove “some of” the droplets from your breath.
“We do think this virus is breathed out in droplets,” she said. “Whether or not those droplets are large or small, it’s quite likely that a face mask will remove some of them from your breath.”
But she said air conditioning won’t always help because some units simply “push the air around”.