Video report by ITV News Consumer Editor Chris Choi
Royal Mail has asked environmentalists posting their crisp packets back to manufacturer Walkers to use envelopes.
Campaigners at 38 Degrees have been encouraging their supporters to post their crisp bags back to the company in protest of them being non-recyclable.
Royal Mail is obligated to forward on the crisp packets to Walkers by law, but without an envelope the empty snack bags cannot be put through sorting machines and must be retrieved by hand.
The postal service has complained this can cause delays.
"We strongly encourage customers not to post anything into the postal system which is not properly packaged," a Royal Mail spokesperson has 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."
Crisp packets take around 75-80 years to decompose.
Cathy Warren, a campaigner at 38 Degrees said: "Royal Mail has asked people to use envelopes when posting crisp packets and we will update the thousands of Walkers's customers who are taking part.
"Up and down the country, people are telling Walkers to step up when it comes to plastic waste.
"Walkers produce a staggering 7,000 plastic crisp packets a minute which they don't pay a penny to clean up. They need to listen to their customers and take action now."
Retired teacher Geraint Ashcroft started the campaign against Walkers after reading on his local council website that crisp packets were non-recyclable.
“I've been shocked by headlines showing packets found still intact after 30 years. This has a horrific effect on our environment," he said.
"We’ve already seen big companies and Government step up and tackle the plastic bag problem and introduce recyclable cups, manufacturing giants like Walkers now need to do their bit.
"Whether it’s Walkers who acts first to eliminate plastic waste, or one of their competitors, there will surely be a huge boost for those companies who take bold action.
"It won’t cost consumers a penny to get involved but if crisp manufacturers don’t take action, they’ll lose a packet.”
Responding to the criticism, Walkers said: “We recognise the efforts being made to bring the issue of packaging waste to our attention.
“The returned packets will be used in our research, as we work towards our commitment of improving the recyclability of our packaging.”
Many have taken photos of themselves posting their crisp packets back to Walkers and shared them on social media using the hashtag #PacketInWalkers.
Speaking to ITV News, environmentalist Jarred Livesey, who has already sent crisp packets back to Walkers, said: "I will continue to return packs to Walkers - I’ll just use a recycled envelope."
"Mum & I are returning our PLASTIC bags back to @walkers_crisps today using the FREEPOST address.. their refusal to ditch plastic until 2025 = 28 BILLION MORE plastic packets will end up in landfill before then.. thank u @38_degrees for this Campaign," wrote another supporter of the campaign, Kate Gibb.
A member of staff at Royal Mail had been confused when sorting crisp packets and joined in with the discussion.
"Aaaaaah that makes sense as to why I sorted (work for Royal Mail ) crisp packets yesterday & today! Did make me wonder......! #PacketInWalkers," wrote Twitter user Sazzle Dazzle.
Walkers plan to make their packaging recyclable by 2025 - alongside other snack manufacturers such as KP snacks, who own McCoy’s, Tyrell’s, Hula Hoops, as well as major supermarkets like Tesco, Asda and Sainsbury's.