Wear face masks while out Christmas shopping on crowded streets could slow the spread of Covid

        Current rules only require the coverings to be worn inside shops, with many quickly ripping them off as soon as they step back outside. But leading experts have now urged Brits to keep their masks on in crowded outdoor spaces, such as packed high streets.

And London's mayor Sadiq Khan called for face coverings to be made mandatory in busy outdoor settings after images showed the capital's streets were packed full of Christmas shoppers. 

His comments came before it was revealed that London would be plunged into Tier Three because of a spike in Covid-19 cases. The toughest curbs will come into force from midnight, with pubs and restaurants forced to switch to takeaway only.

The city of nine million became the country's coronavirus hotspot last week, after recording an infection rate of 242 cases per 100,000 people in the seven-day spell ending December 10. Professor Trish Greenhalgh, a primary care expert at the University of Oxford, told The Times that face masks should be worn when people are packed close together.

'In addition to "shared air" (mostly an indoor risk), there's also the problem of being caught in the direct jet of an exhaled gas cloud when someone coughs, speaks, sneezes or just breathes out,' she said.

 If you're close enough to feel their warm breath on you or smell their halitosis (bad breath) you probably need a mask even outdoors. Professor Melinda Mills, a sociologist also at Oxford University, added that Briton was an outlier in insisting masks should only be worn inside - and not outside in public.

'When we looked at the data in June, there were 71 countries who required everyone in public to be masked,' she said. It was a minority of countries that chose specific complicated policies. (The virus) is aerosol, it's transmitted in air, this is good to do.'

Mr Khan has backed the calls, saying yesterday: 'Face coverings should also be made mandatory in busy outdoor public spaces, given the numbers on our high streets in the run-up to Christmas.'

He told BBC Breakfast this morning that the London Assembly has been lobbying the Government to make the change to its rules. 

The World Health Organization suggested people living in high transmission areas should wear face masks outdoors when they can't keep a metre apart at the start of this month.

But they admitted there was 'limited evidence' that the masks halt transmission of the virus.  In their report, they also suggested the coverings should be worn in private homes if someone comes inside who does not live there.

A spokesman for the Department of Health told MailOnline: 'It is mandatory for face coverings to be worn on public transport and in a range of indoor public settings where social distancing may be difficult and where people come into contact with others they do not normally meet. There is strong scientific evidence that the risk of transmission is higher in enclosed spaces compared to outdoors, and it is this risk of transmission that wearing a face covering will help reduce.