A postal worker in Welwyn Garden City has raised concerns regarding extra work making deliveries "unachievable".
The postman, who did not wish to be named, claimed delayed mail was imminent as managers have "no idea how to implement a fair and achievable days' work."
He added: "WGC are not getting the universal service and at best post is delivered every other day as managers tell staff to prioritise tracked items."
A Royal Mail spokesperson said: “Over the last 10 years, the number of addresses we deliver to nationally has grown by more than two million, and many postmen and women’s routes have not been revised in this time.
"This can mean that some routes can be an hour or two longer than others. Our latest quarterly results show a 25 per cent decline in the volume of letters sent since before the pandemic, during which time the number of parcels has risen.
"We have therefore revised routes throughout the country to ensure they are fairer and reflect the growth in addresses and parcel deliveries. We are undertaking such a revision in Welwyn Garden City, and do not anticipate any delays locally as a result."
The WGC postal worker added: "Going forward the service will continue to be very poor due to complete decimation of a once great service delivered with pride by loyal staff.
"Royal Mail is on a path to self-destruct and bring the Universal Service Obligation (USO) to its knees so it can be palmed off to concentrate on the profits of parcels."
The spokesperson added: "We do not operate a policy of prioritising parcels, and regularly remind colleagues that the delivery, collection and processing of letters and parcels should be treated with equal importance.
"Due to the amount of space, they take up, parcels can restrict the movement of both people and mail in our offices, leading to health and safety issues and delays to other mail.
"At particularly busy times, such as in the recovery days after a strike, we may occasionally at a local level clear parcels to free up space and address health and safety concerns so that we can keep all mail moving efficiently through our network."
same in our office