Watch the drone footage taken outside the Bristol Mail Centre
Footage taken outside Bristol Mail Centre shows stacks of undelivered post stacking up in sorting cages alongside stationary delivery trucks.
With Christmas little more than a week away, the strike is likely to only increase the pile-up.
The piles in Bristol have even attracted a fox and rats, with one Royal Mail worker saying they have never witnessed a backlog like this before.
There are two more strike dates planned before Christmas unless Royal Mail bosses change their current offer, which The Communication Workers Union (CWU) says involves worsening terms and conditions alongside a below-inflation pay award.
Ben, who has been working for the CWU for over 17 years, said: “This backlog will take at least a month to clear.
"When Royal Mail says, ‘everything is okay’ it’s absolutely not true. The work’s coming in, they’re on-loading it but there’s nowhere to put it.
“If you post a first-class letter or parcel today, hand on heart, I do not know if it will get there before Christmas Eve, that’s the truth but it’s not what people are being told."
A Royal Mail spokesperson said: "The CWU is striking at our busiest time, deliberately holding Christmas to ransom for our customers, businesses and families across the country.
"We are doing everything we can to deliver Christmas for our customers, and would like to thank the increasing number of posties returning to work each strike day, temporary workers and managers from across the business who are helping to keep the mail moving.
"However, this task becomes more challenging as Christmas nears.
"Three weeks ago, we made a best and final pay offer worth up to 9% over 18 months.
"Instead of working with us to agree on changes required to fund that offer and get pay into our posties’ pockets, the CWU has announced plans to ballot in the New Year for further strike action.
"Their misguided belief that further industrial action - in a business already losing more than £1 million a day - will result in an improved pay offer is misleading its members and risking their long-term job security."