London Mayor Sadiq Khan arrives in Downing Street in London ahead of government briefing on March 16. Peter Summers/Getty Images

The Mayor of London Sadiq Khan called for masks to be worn on public transport and in shops, in a Friday interview with the BBC.

Khan said the UK is an "outlier" for not recommending masks and called for non-medical facial coverings such as bandanas, scarfs and reusable masks to be worn in public places.

"I'm lobbying our government's experts and our government to change the advice," Khan said. "If you really can't stay at home, if you really have to use public transport and you can't keep your social distance, then wear a non-medical facial covering."

The UK government guidelines on social distancing do not mention masks or facial coverings. In a Downing Street press conference on April 3, the Deputy Chief Medical Officer Jonathan Van-Tam said there is "no evidence that general wearing of facemasks by the public who are well effects the spread of the disease in our society."

The World Health Organization is standing by its recommendation to only wear a mask if you are sick or caring for someone who is sick.

According to the WHO website: "If you are healthy, you only need to wear a mask if you are taking care of a person with suspected 2019-nCoV infection." It also says: "Masks are effective only when used in combination with frequent hand-cleaning with alcohol-based hand rub or soap and water."

But a growing number of countries have nonetheless updated their advice, urging their citizens to wear masks if they leave the house.

In a series of Friday tweets, Khan added, "It breaks my heart that 20 London bus workers have lost their lives to #COVID19. It could easily have been my dad & his friends. I‘m taking part in this minutes silence at 11am with @UnitetheUnion and urge all Londoners to join us."

"Today I announced additional action to keep drivers safe, with middle-door only boarding on buses from Monday on top of enhanced cleaning & new protective screens on drivers’ cabs. Our transport workers are heroes and we must do everything we can to protect them," Khan said.