The Prime Minister said the move which also applies in shops has been introduced as a precaution while more information is gathered and assessed on the impact of the variant. The new mask mandate will be reviewed in three weeks, he said.

Travel watchdog Transport Focus chief executive Anthony Smith said: “Most passengers will understand why this step has been taken. Transport operators must give clear information about wearing face coverings so nobody can be in any doubt.

“It’s also important to acknowledge some people are exempt,” he added. “In our most recent survey, before the rule changed, half of rail and bus passengers were satisfied with the number of people wearing face coverings – we expect that proportion to now increase.”

Evidence suggested that growing numbers of public transport users were not wearing face masks prior to the discovery of the Omicron variant. London TravelWatch director Emma Gibson said: “Research shows that for many Londoners, other people wearing face coverings is a key factor in making them feel safe about travelling on our buses and the Tube.

“The number of people wearing a face covering on public transport has dropped significantly over the past few months. Making them a requirement on all public transport again will be reassuring for many people but it will have to be properly enforced to give out the signal that the rules have changed.”

The Confederation of Passenger Transport which represents bus companies said in a statement that operators would be reminding passengers of the legal requirement to wear a face covering unless exempt.

“As they have throughout the pandemic, we expect passengers will work with operators to ensure they can travel safely,” the group said. “People can still have the confidence to travel by bus with measures to improve ventilation and tools such as apps to allow customers to see how busy their bus is.”

A spokesman for the Rail Delivery Group said: “Rail companies are helping people to travel with confidence when they take the train by working with Government to promote new measures that came in to effect in England this week.

“Across the railway, people must wear a face covering, unless exempt, and while we know the vast majority of people will want to do the right thing, we’ll be working with British Transport Police who will encourage and enforce the measures.”

Transport workers’ unions have warned that the mask mandate must be enforced strongly. RMT general secretary Mick Lynch said: “We support the wearing of masks but there are major issues about enforcement. The Government must make the resources available to properly police this reintroduction of compulsory face coverings on our transport services.”

Unite national officer for passenger transport Bobby Morton added: “It is not sufficient to announce that face masks will once again become compulsory, this policy has got to be fully enforced in order to protect public health.

“The Government’s previous inconsistent messaging on face mask wearing is almost certainly going to result in a high degree of non-compliance.”

British Transport Police said it was “confident passengers will play their part in protecting each other” and officers will continue taking a ‘Four Es’ approach of “engage, explain, encourage and, as a last resort, enforce the legal requirements”.

(Photograph: Transport for London)