The ex-Royal Mail employee, who wishes to remain anonymous, called the situation a "shambles" as letters and untracked parcels are regularly delivered weeks late, particularly around Moor Lane, Padiham, and the Sycamore area of Burnley.
The distribution giant is under a universal service obligation by the Government to deliver mail to every address in Britain, every Monday to Saturday.
“It’s a complete shambles. The [staff members] want to do a good job. I wanted to do a good job. It’s frustrating.
"I feel sorry for the people of Burnley. They need to know what is going on with the mail. It’s disgusting."
He claims a number of staff members have left the Burnley division in the past seven to eight months due to "unmanageable" workloads, which have increased from four hours to five to six per round of deliveries as colleagues absorb additional postal areas.
"Staff [members] have unmanageable workloads. We are out on foot and given the same time to do [the extended rounds]. [The bosses] know it is not doable.
"They are not giving you [an additional round] a couple of streets away: they are giving you something totally out of the way. It doesn’t make sense at all.
“Since the new bosses came in seven or eight months ago, staff have been leaving left, right, and centre."
A Royal Mail spokesperson said the route changes reflect the two million new addresses gained in the past decade, plus a 25% decline in people sending letters and a rise in parcels since before the pandemic.
But the company is "confident" it has the resources to address its anticipated workload.
The former postman, who worked for Royal Mail for at least a decade, says bosses instruct staff to concentrate solely on special deliveries and packages that customers can track online. Meanwhile, first and second-class letters and parcels are put "on the back burner."
Mail "stacks up and up" at Burnley Delivery Office - even if "it’s from the NHS."
And if mail is delivered, it is regularly late, he adds, with his mother-in-law receiving three weeks' worth of letters - about 30 - all at once. Christmas cards were two months late.
A Royal Mail spokesperson claims the company is "always very sorry to hear reports of delays and would apologise for any inconvenience caused."
They added it would only prioritise parcels for health and safety reasons and regularly reminds colleagues that all kinds of mail are equally important.
The ex-postie adds the situation is now "twice as worse" as when he left his job around three months ago due to these issues.
“I didn’t leave on bad terms. I loved my job. But I’ll never use Royal Mail again because of the way it is run. I wouldn’t trust it."
However, some 97% of customers and small businesses participating in research by communications regulator Ofcom say they "no longer need" mail delivered on Saturdays and that a five-day service would be enough.
A Royal Mail spokesperson says its delivery obligation requires "major reform" by the Government to be financially sustainable as it comes at a "significant structural cost".