Transport for London staff are “frustrated” there is no requirement for passengers to prove they are exempt from mandatory mask wearing.

While around 90% of passengers comply with the rules, according to TfL, approximately 137,000 people have been told to wear a covering since the introduction of rules June last year.

Speaking to the BBC, Director of Director of Compliance, Policing, Operations and Security, Siwan Hayward, said the nature of the law means staff must rely on their communications skills.

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If a passenger says they are exempt, staff members have to take them at their word.

Siwan Hayward: "That's the way the law is constructed.

“Some of my colleagues are frustrated that there's not a requirement to prove you're exempt.

"However, my officers in TfL are all very well trained at getting honest answers out of people and will be able to make a judgement.

"We're pretty confident the system is working pretty well."

TfL said about 90% of its passengers wear masks and most passengers do put on their masks when asked to do so by enforcement officers, TfL said.

The BBC reports that more than 9000 people were prevented from boarding and a further 2,100 people were removed from services for not wearing a mask.

Those 2,100 people were then issued £200 Fixed Penalty Notices by TfL officers.

Staff at stations are not required to wear masks while on duty, although it is encouraged in order to set a good example.

City Hall and TfL are reportedly reviewing rules in the capital, considering whether the use of high-filtration masks, in place of cloth coverings, should be encouraged.

The disposable masks, commonly blue, are harder to breathe through, offering extra protection against the spread of more contagious strains.

Scientific advice suggests, that, although face coverings are unlikely to prevent an individual from catching the coronavirus, they can help prevent someone who is infected from infecting others and therefore help control the virus.