Campaigners are sending empty packets to Walkers to protest its non-recyclable packaging - but waste slows down sorting.
Royal Mail has urged campaigners to stop sending crisp packets without envelopes to the headquarters of Walkers in the post.
People have been sending empty packets with sticky labels on to the snack maker to protest at the use of non-recyclable plastic as part of a campaign backed by online activism group 38 Degrees.
The campaign, started by Geraint Ashcroft, provided instructions for securing an address label to an empty packet and sticking it in the post, and urged participants to post photos of their protest on social media.
"By posting our packets to them, we can bring the crisis right to Walkers door," it said. "Walkers bosses won't be able to escape our call."
But Royal Mail is telling protesters they should always use proper packaging when sending items in the post.
It has "strongly encouraged" people to refrain from posting the packets, which it is legally obliged to mail to Walkers' freepost address.
"We strongly encourage customers not to post anything into the postal system which is not properly packaged," a spokesperson said.
"Crisp packets can't go through the machines, they are not normal mail items therefore my hardworking colleagues need to manually sort them, which adds to time."
Hundreds of thousands of people have signed a 38 Degrees petition calling on Walkers to stop using plastic packaging.
Although Walkers has committed to make its crisp packets biodegradable, compostable or recyclable by 2025, 38 Degrees says that's not good enough.
The site's own research has found that by that year Walkers is likely to produce a further 28 billion crisp packets.