Royal Mail has stepped into a row between campaigners and crisp makers Walkers - by urging people posting empty packets to put them in envelopes. Campaigners asked people to post the non-recyclable plastic bags and "flood Walkers social media with pictures of us popping them in the post". Royal Mail is obliged by law to deliver the bags to Walkers' freepost address. But without envelopes they cannot go through machines and must be sorted by hand, causing delays. A Royal Mail spokesperson said: "We strongly encourage customers not to post anything into the postal system which is not properly packaged. "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."