Traces of the coronavirus found in a hospital air duct has led scientists to believe the disease could be spread through air-conditioning units, making it more contagious than initially thought.

Swab analysis of rooms used by three coronavirus patients by experts at the National Centre for Infectious Diseases in Singapore suggest that the respiratory illness spreads easier than previously thought.

An air duct connected to the room of one of the patients, thought to be only suffering from 'mild' symptoms', was found with traces of the virus, suggesting 'suggests small, virus-laden droplets may be displaced by airflows and deposited on equipment such as vents'.

The research, published in the Journal of the American Medical Association, comes shortly after it was reported that 142 Britons were reported trapped on a Princess Cruise ship anchored for patients to undergo testing, off the coast of California yesterday.

Pictured: File photo of an air-conditioning unit.

The Diamond Princess, pictured in Yokohama last month, became one of the world's largest clusters of coronavirus cases when more than 700 people were infected on board

Health authorities have had a suspicious eye on cruise ships in recent weeks ever since a number of voyages were disrupted by virus scares.

In the most serious case, 705 people tested positive for the virus on board the Diamond Princess during a two-week lockdown in Japan.

Japanese authorities said last week that a British tourist who had been on board the Diamond Princess had died after contracting coronavirus.

On Thursday, a patient with underlying health conditions became the first person in the UK to die after also testing positive for the virus.

It is standard practice for buildings and cruise ships to use recycled air through air-conditioning systems.

Coaches carrying British evacuees from the coronavirus-hit cruise ship in Japan arrive at Arrowe Park Hospital in the Wirral after they were flown home

Professor James G. Dwyer, from Purdue University in Indiana, told the Telegraph: 'The problem is that these systems can't filter out particles smaller than 5,000 nanometers.'

The size of the coronavirus is not yet known, but a similar respiratory illness, Sars, was recorded at just 120 nanometres.

Professor Dwyer said that if the current COVID-19 is a similar size, 'the air conditioning system would be carrying the virus to every cabin.

'Cruise ships could minimise this problem by just using outside air and not recirculating it,' he added.

Airlines have already sought to reassure passengers that their air-conditioning systems are fit to prevent the coronavirus spreading in the cabin, with Etihad suggesting their system is as watertight as a hospital operating theatre.

'In any confined area, there is a risk of contracting illnesses from other people,' an airline statement reads.

'However, the risk is considered lower on aircraft because of the use of high efficiency particulate air filters, which are effective in capturing more than 99 per cent of airborne microbes in filtered air.'