Prime Minister Boris Johnson has promised to keep the 2m (6ft) rule for social distancing "under constant review".

It comes after increasing pressure from MPs and the hospitality industry to cut it to 1m to help businesses after they reopen.

However, scientists continue to question whether that would be safe, given how little is known about how far coronavirus can spread.
What does the science say?

The simple answer is that the nearer you are to someone who is infected, the greater the risk of catching the virus.

The World Health Organization says that a distance of 1m is safe. Some countries have adopted this guidance, while others, including the UK, have gone further:

1m distancing rule - China, Denmark, France, Hong Kong, Lithuania, Singapore
1.4m - South Korea
1.5m - Australia, Belgium, Germany, Greece, Italy, Netherlands, Portugal
1.8m - US
2m - Canada, Spain, UK

It's not just about distance

Timing is also key. The longer you spend in close proximity with an infected person, the bigger the risk.

Scientists advising the UK government say that spending six seconds at a distance of 1m from someone is the same as spending one minute at a distance of 2m.

Being exposed to someone coughing is riskier. Being 2m away from a cough carries the same risk as someone talking to you for 30 minutes at the same distance.
What's the latest research?

In a study published in the medical journal The Lancet, scientists evaluated recent research into how coronavirus spreads.

They conclude that keeping at least 1m from other people could be the best way to limit the chances of infection.

The risk of being infected is estimated to be 13% within 1m, but only 3% beyond that distance.

And the study says that for every extra metre of distance up to 3m, the risk is further reduced by half.

Where does the distancing rule come from?

It can be traced back to research in the 1930s.

Scientists found that droplets of liquid released by coughs or sneezes evaporate quickly in the air or fall to the ground.

Most of those droplets, they reckoned, would land within 1-2m.

That is why it is said the greatest risks come from having the virus coughed at you from close range, or from touching a surface that someone coughed onto, and then touching your face.

Can the virus travel further in other ways?

Proximity and surface contact are considered the main transmission routes.

But some researchers fear coronavirus can also be transported through the air in tiny particles called aerosols.

If true, then the flow of wind from someone's breath could carry the virus over longer distances.

Prof Lydia Bourouiba from the Massachusetts Institute of Technology (MIT) used high-speed cameras to capture a cough projecting miniature specks as far as 6m.

And a study carried out in Chinese hospitals which found traces of coronavirus in Covid-19 wards and intensive care units, estimated that 4m was a better safe distance.

But the US Centers for Disease Control says the role of aerosols in spreading the virus is "currently uncertain".

And what's still not known is whether any virus that spreads further than 2m can still be infectious.

Coronavirus: Can we stay safe as lockdown eases?

What else makes a difference?

It is widely accepted that the infection is more easily passed on indoors than outside in the fresh air.

Japanese researchers investigated 110 cases of Covid-19, following up the contacts of those infected.

They estimated that the odds of the infection being passed on were nearly 19 times greater indoors than outside.

In many countries, including England and Scotland, people are being encouraged to wear face coverings on public transport and "enclosed spaces where social distancing is not always possible".

Why don't we have definite answers?

It is only a few months since the coronavirus emerged, and in that short time scientists have learned a great deal about it.

But there is a long way to go, and confirming exactly the right distance is one of the unanswered questions.

Answering it will require careful studies of how the virus can be carried, and how viable it remains, which will all take time.