Royal Mail said affected parcels have been held back for "examination and deep cleaning" by a specialist contractor.
Royal Mail has had to deep clean parcels at one of its depots after a rat infestation.
Operations at the Parcelforce Worldwide depot in Maesglas, Newport, have "temporarily moved" after reports some parcels were contaminated with rat urine and droppings.
Affected parcels have been held back for "examination and deep cleaning" by a specialist contractor.
Royal Mail, who operate Parcelforce Worldwide, said the "localised issue" had been "identified and addressed promptly" through "deep cleaning and specialist pest control action".
They added the parcels will be released to customers once it is safe to do so.
An anonymous source told BBC Wales parcels within the depot have been "contaminated with rat urine and droppings, and some have been visibly chewed".
An anonymous source told BBC Wales the "significant rat infestation that management has failed to properly resolve" had been going on for several months.
They claim that rats are "frequently seen across the site" and "chewed through" the fuel injector lines of one delivery van.
The source said the AA came to look at the van and said "you have a rat problem in there don't you?". The van was taken off the road to be fixed following the findings, they said.
"Despite this, management continues to allow these vehicles to be used, putting drivers at risk," the source alleged.
"Parcels within the depot have also been contaminated with rat urine and droppings, and some have been visibly chewed," they added.
After the issue was raised, "limited action" has been taken, but the infestation "remains ongoing", according to the source.
A spokesperson for the Royal Mail said claims that "limited or insufficient action has been taken" were incorrect.
They added they had mobilised specialist support, including baiting and trapping and proofing work to prevent further access, and were continuing to take all necessary steps to resolve the issue as quickly as possible."
Rats carry several serious diseases potentially transmissible to humans, which are spread through urine, droppings, as well as parasites such as fleas, ticks, and mites.
Elsewhere in Newport, there has been reports of rats "the same size as a rabbit" in Pillgwenlly due to ongoing problems with fly-tipping in the city.
A Royal Mail spokesperson said: "We are actively managing a pest issue at our Newport depot and have taken immediate steps to address it.
"While there has been evidence of rodent activity in parts of the site, this has been largely localised and we have moved operations to an unaffected area of the depot while work is carried out."