Royal Mail is putting "profits before safety" say postal workers, who claim the company is failing to protect them from the risk of catching coronavirus.
There is a shortage of gloves, masks and hand sanitiser, employees from across the UK have told the BBC.
They also claim social distancing at work is "almost impossible".
Royal Mail says it has invested £15m in protective equipment and that the health and well-being of staff is their top priority.
Over the last two weeks, staff at eight Royal Mail sites, including three in Scotland, have walked out over safety concerns. The BBC has seen footage of employees working shoulder-to-shoulder in one sorting office, with limited social distancing measures in place.
A supervisor based in the north of England said: "It's more than two weeks since the lockdown and we're in a situation where there is still a lack of PPE [personal protective equipment], there's still a lack of sanitisers, there is still a lack of direction."
He said staff feel as though they are "forced to choose between their jobs and their health".
"I'm scared that my job will be in danger if I refuse to do a task because I believe it puts me in an unsafe position," he added.
Since the lockdown began there have been unofficial walk-outs at Royal Mail sites in Chatham, Southampton, Stoke-on-Trent, Warrington, Didcot, Edinburgh, Alloa and Fife.
Delivery and sorting office staff based in the North West and the Midlands have described difficult conditions with "no space" to keep safely two metres apart. They too feared for their jobs if they raised concerns publicly.
One postman said: "The buildings we work in aren't designed for people to be able to space out. Everyone's gathered together. We're under each others' skin. It's just completely impossible".
He also criticised the company's response after he raised worries about insufficient supplies of hand sanitiser during delivery rounds.
"When this was all kicking off, we were saying [to management] we need this stuff. But our big boss upstairs just said 'no you just need to wash your hands'. I'd love to wash my hands, but nowhere's open. Everything's shut. It's ridiculous."
Another postman said: "We are touching 850 letterboxes with no protection. Coronavirus is spoken about like a nuisance at my office."
"It's not too much to ask to be equipped for the job," he added.
The Communication Workers Union (CWU), which represents thousands of postal workers, accuses Royal Mail of being slow to act in response to the crisis.
North West divisional rep Ian Taylor said it had been "busier than Christmas" with parcels and said it was imperative that protective equipment reached frontline staff.
The CWU estimates that 20% of Royal Mail staff are currently off work. That equates to around 26,000 postmen and postwomen who are either sick or self-isolating, at a time when parcel deliveries have soared as more people shop online during the lockdown.
"People really do need to think about what they are buying at this particular time, it's placing incredible strain on frontline posties," Mr Taylor said.
Postal workers told the BBC that they were 'exhausted' by the extra workload, delivering items like home gym kits and garden furniture, as well as junk or advertising mail.
To ease the workload, Royal Mail has changed its policy on junk mail. It will now only be delivered to people already receiving letters or parcels.
The company refutes the union's claims and says "wherever possible" workers were being kept least two metres apart. Regular handwashing with soap and water is promoted amongst staff and there is "enhanced disinfectant cleaning of communal areas." The company has bought 400,000 bottles of hand sanitiser, it said.
A Royal Mail spokesperson said: "In assessing the risks to our people and making the necessary operational changes to protect them, we take professional medical and health and safety advice on a daily basis.
"We have already made a series of adjustments to our parcel-handling procedures to protect our colleagues and our customers."