Downing Street has insisted its hotel quarantine policy is “in line” with other countries, despite being warned by an Australian epidemiologist that allowing travellers to leave their room for fresh air is “very risky”.

Professor Michael Toole, from the Burnet Institute in Melbourne, Victoria, said rules in Australia had to be strengthened to reduce the chance of airborne transmission – suggesting England should adopt similar measures.

He 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 nationals or residents returning to England from 33 “red list” countries will be required to spend 10 days in a Government-designated hotel from Monday but, unlike in Australia, they will be allowed to leave their rooms for fresh air.

Unite the union said it was “deeply alarming” for hotel workers that the guidelines are “far inferior” to those in Australia.

Hospitality organiser Bryan Simpson said: “Once again the Government has been found guilty of prioritising headlines over the safety of workers.

“It must not be ignored that workers are in as great a danger of transmitting the virus between each other as being exposed to it from quarantining passengers.”

It will be up to individual hotels to decide on the exercise rules, but the Department for Health dismissed a report that travellers would be allowed outside their rooms to smoke.

They also insisted hotel staff will have access to regular testing and suitable PPE.

Downing Street defended the level of restrictions, with a spokesman telling a Westminster briefing: “They are in line with other countries who are taking this approach.

“We require repeat testing, travellers to quarantine inside their room for 10 days and we have strict penalties in place for anyone who fails to comply.”

He said travellers are allowed outside for exercise “with permission from hotel staff”, but stressed that the list of exemptions to staying inside the hotel room is “quite limited”.

They are in line with other countries who are taking this approach

The spokesman also said additional hotel rooms can be added “as required” after signing contracts for 4,600 rooms so far.

Home Office minister Victoria Atkins earlier said it was “reasonable” to allow travellers quarantining in hotels a “gulp of fresh air”.

She told Today: “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.

“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.”

Ms Atkins added: “We are confident that the measures that we have in place, ready to go on Monday, are strong and that they will help to protect our country against any of these new variants that are being found.”

Travellers from countries on the banned list can only arrive into one of five airports in England.

Guidance says that anyone from one of those countries with a booking that brings them to a different “port of entry” from February 15 must change it to one of those specified.

The accepted entry points are: Heathrow Airport, Gatwick Airport, London City Airport, Birmingham Airport and Farnborough Airfield.

The published guidance says that leaving the room for exercise will only be allowed with special permission from hotel staff or security and is “not guaranteed”.

One of the Heathrow Airport hotels taking part in the scheme is Novotel London Heathrow T1 T2 T3.

A spokesman for owner Accor said: “Novotel London Heathrow T1 T2 T3 has answered the call from Government to assist with the mandatory hotel quarantine in order to support the safe return of Brits.

“Covid Government policy is designed to control the virus and keep people safe and our role is to support that action.”

The hotel is charging £65 for members of the public staying on Sunday night, while travellers using it to quarantine from Monday must pay £1,750 for 10 days.

In Scotland, where all arrivals must isolate, six hotels near to Aberdeen, Edinburgh and Glasgow airports have been block-booked to house travellers quarantining, with a combined total of up to 1,300 rooms available.