The UK government has insisted its hotel quarantine policy is in line with other countries after an Australian epidemiologist warned that allowing travellers to leave their room for fresh air is "very risky".
Professor Michael Toole from the Burnet Institute in Melbourne suggested the UK could learn from his country, where preventative measures initially focused on stopping the spread of coronavirus by large droplets but had to be strengthened.
He told BBC Radio 4's Today program that the precautions - such as wearing surgical masks, keeping people in their room and using hand sanitiser - did not prevent airborne transmission.
Toole said there have been COVID-19 cases in the city where an infected guest opened their room door and "with the positive pressure this kind of fog of virus went out into the corridor, travelled down and infected hotel staff".
Asked for his views on people being allowed to leave their rooms in England's quarantine hotels while accompanied by guards, he said: "We've learnt that that is a very risky procedure."
UK citizens or residents returning to England from 33 "red list" countries will be required to spend 10 days in a government-designated hotel from Monday.
However, the rules are more relaxed than those in Australia, where people are not allowed to leave their rooms.
The BBC said the government's official requirements for hotel operators state that security staff can "accompany any of the arrived individuals to access outside space should they need to smoke or get fresh air".
But the Department of Health denied people will be allowed outside to smoke, though it said it would be up to hotels to decide on exercise rules.
Downing Street later insisted the restrictions at quarantine hotels are "in line with other countries who are taking this approach".
Home Office minister Victoria Atkins defended the policy, saying it is "reasonable" to allow travellers quarantining in hotels a "gulp of fresh air".
"The hotel will of course be adhering to all of the very strict measures that we have in place in relation to social distancing and face masks and so on," Atkins said.
"So I think allowing someone a gulp of fresh air, apart from anything else, we know that being outside is less likely to transmit than being inside.
"But I think allowing someone a gulp of fresh air during a 10-day visit in a hotel, with all the very strict measures that we have, I think is reasonable - but of course we will keep these measures under review."
But Unite, a union, said it was "deeply alarming" for hotel workers that the guidelines are "far inferior" to those in Australia.