Only 20 per cent of people are still wearing face masks on public transport now they are no longer a legal requirement, official figures have revealed.
Network Rail said that one in five rail passengers were continuing to wear face coverings in England now that it was no longer compulsory, compared to 80 per cent before "Freedom Day" on July 19.
There is no legal requirement to wear one on public transport in England, although it remains the case in Scotland, Wales and Northern Ireland.
Transport for London (TfL) has retained mask-wearing as a condition for travel on buses and Tubes in London.
A government statement said: "The guidance is clear that people are expected and recommended to wear a mask when they come into contact with people they don't normally meet in enclosed and crowded spaces."
"It is open to transport operators to decide if they want to implement their own policies, working within their particular environment."
Several union chiefs who represent transport workers have said they are concerned about the decrease in mask-wearing, even on TfL services.
The unions also fear it would be difficult to reintroduce the policy as mandatory if the Government decided fresh restrictions needed to be implemented.
Mike Lynch, the General Secretary of the RMT union, said members of his union had noticed the drop in mask-wearing since the Government eased Covid restrictions, including on TfL services.
He said: "The policy is coming apart at the seams and as more people see others failing to comply, the situation will escalate quickly over the autumn."
He added that there is a "serious issue over enforcement", meaning transport workers were "potentially put into the front line" and were at risk of abuse if people confronted them.
"With the Government already making compulsory mask-wearing on transport a contingency if cases escalate in the coming weeks, there is a real danger they won't be able to get the genie back into the bottle," Mr Lynch said.
A spokesperson for Unite, which represents bus drivers, said: "Our members are reporting that mask-wearing on buses is collapsing and has gotten much worse in recent weeks.
"In some cases, there are very few passengers wearing masks on a bus, with those who tend to still wear a mask being older.
"For our members, while they are relatively safe when actually driving the bus because of the sealed cabs, there is growing concern that they are being placed in danger when they are required to board a bus at a depot to be taken to a rendezvous point to collect the bus they are allocated to drive."
The General Secretary of the train drivers' union Aslef, Mick Whelan, urged the Government to make mask-wearing mandatory on trains to "protect passengers and staff".