The two-metre social distancing rule is a “random measurement” and does not protect against catching Covid, a University of Cambridge study has suggested.

Researchers said that the number was chosen from a risk “continuum”, rather than being a mark of safety, and without extra measures, such as face masks, a person with Covid could infect someone else at a two-metre (6.5ft) distance.

The Government first advised the public to stay two metres apart in March 2020 to prevent the spread of Covid.

That June, the rule moved to “one-metre plus”, which allowed people to remain one metre (3.3ft) away if extra measures were in place, such as face masks or meeting outdoors.

The Cambridge team used computer modelling to measure how droplets spread between people through coughing, including how the droplets flow through the air and evaporate.

Published in the journal Physics of Fluids, the findings showed that there is no sharp cut-off once the droplets spread beyond two metres and smaller droplets in the air can easily move beyond this mark.

Two-metre rule an ‘easy to remember message’

“Each time we cough, we may emit a different amount of liquid. So if a person is infected with Covid-19, they could be emitting lots of virus particles or very few, and because of the turbulence they spread differently for every cough,” said Dr Shrey Trivedi, of the university’s department of engineering, and the study’s first author.

Prof Epaminondas Mastorakos, who led the research, added: “Even if I expel the same number of droplets every time I cough, because the flow is turbulent, there are fluctuations. If I’m coughing, fluctuations in velocity, temperature and humidity mean that the amount someone gets at the two-metre mark can be very different each time.”

The researchers said that the two-metre rule is an easy to remember message, but should not be taken as a sign of safety.

“We’re all desperate to see the back of this pandemic, but we strongly recommend that people keep wearing masks in indoor spaces such as offices, classrooms and shops,” Prof Mastorakos said. “There’s no good reason to expose yourself to this risk as long as the virus is with us.”

Social distancing rules were relaxed in England in July, including dropping the two-metre rule in hospitals to one metre.