Anyone travelling on public transport in England must wear a face covering from Monday under a new rule.
More than 3,000 extra staff including police officers are being deployed at stations to make sure people comply.
Authorities will ask passengers without a covering to wear one or they will face being refused onboard or fined £100.
People with certain health conditions, disabled people and children under the age of 11 will be exempt from the rule.
In the coming days, hundreds of thousands of free coverings will be handed out at railway stations. The government says masks can be homemade, such as a scarf or bandana.
As well as on transport, all hospital visitors and outpatients also have to wear masks.
The UK government changed its advice on face masks to stop the spread of coronavirus earlier this month, as more people used public transport to go back to work. It is now in line with the World Health Organization’s advice.
New compulsory rules only in England
The new rules apply to England and require anyone travelling by bus, coach, train, tram, Tube, ferry or plane to cover their face while on board.
They exclude school transport, taxis and private hire vehicles – although Uber has made face coverings compulsory for passengers and drivers.
The rules apply only while travelling – not while waiting – but the rail industry has asked people to cover their face as they enter a station.
As of today, face coverings are compulsory on public transport in England.
— Department of Health and Social Care (@DHSCgovuk) June 15, 2020
The compulsory rules do not apply in Scotland, Wales or Northern Ireland – but their governments recommend that people cover their faces in places where social distancing is difficult, including on public transport.
Officials expect passenger numbers to reach about 20% of capacity on the railways by early next month.
At those levels, the social distancing of 2m might still be possible, but social distancing will not be possible if passenger numbers increase to around 50% of capacity after the summer as predicted.
Transport Secretary Grant Shapps urged people against using medical masks, saying they must be kept for clinical settings.