Royal Mail has been forced to ask people posting empty crisp packets to put them in envelopes first following the launch of a campaign against plastic waste.
Campaigners have asked people to send the non-recyclable bags back to manufacturer Walkers and put pictures of themselves “popping them in the post” on social media.
But Royal Mail has urged customers not to post anything that is not correctly packaged, warning that such items could be delayed or taken out of the system.
If the packets are not placed in envelopes they cannot go through machines and must be sorted by hand.
A Royal Mail spokeswoman said: “If an item is addressed properly and carries the correct postage then Royal Mail is obliged by law to handle and deliver the item to the stated address.
“We strongly encourage customers not to post anything into the postal system which is not properly packaged. And if they are taking part in this campaign we would urge them to put crisp packets in an envelope before posting.
“Customers can obtain advice from any Post Office branch on packaging and restrictions. Anyone posting items which are not properly packaged could find they are delayed or taken out of the postal system.”
Royal Mail said it had only seen around 30 of the items so far.
People have been taking selfies of themselves posting the crisp packets and putting them on social media with the hashtag #PacketInWalkers.
It follows a petition hosted by 38 Degrees which called for Walkers to make its packets recyclable.
Cathy Warren, 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 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.”
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.”