He said this would reduce the risk of spreading Covid - and increase the confidence of commuters to return to their work places and visitors to return to the West End.
Since the national lockdown rules were eased on July 19, the requirement to wear a mask on the Tube has only been a “condition of carriage” rather than a legal requirement.
This means TfL’s 400 enforcement officers can prevent passengers from boarding or can ask them to leave a bus, Tube or Overground train – but they have no power to impose fines.
Introducing a bye-law would effectively return the rules to the tougher position pre-July 19, when the Met and British Transport Police were able to impose fines and prevent people from travelling without a face mask, unless they were exempt.
Mr Khan told the BBC Newscast podcast with Adam Fleming: “We are trying to lobby the Government to allow us to bring in a bye-law, so it will be the law again, so we can issue fixed penalty notices and we can use the police service and BTP to enforce this.”
He said 86 per cent of passengers continued to follow the rules on mask-wearing.
He said studies by Imperial College had found “not one example” of Covid being present on surfaces or in the air on the Underground and added: “Public transport is safe. What gives people even more confidence is if they know people are wearing face masks.
“We need people to be coming back to the West End. We want to encourage people to return to their offices. They are not going to do so if they don’t feel public transport is safe.
“I’m hoping the Government understands, on the issue of public safety and public confidence, we want to be able to use the law to make sure people do wear face masks in spaces where you can’t keep your social distance for obvious reasons.”