A charity that posted medals to fundraisers was left disappointed when envelopes containing them arrived torn open with just paper inside.
Reading-based charity Parents and Children Together (Pact) said people who took part in its Marathon in May challenge were sent the medals for covering 26.2 miles (42.2km) over that month.
But the charity said at least three of the envelopes had arrived with a corner ripped and the medals missing.
Royal Mail said the envelopes were caught in its mail processing machine because of their "irregular shape" and apologised for any inconvenience.
Pact is a voluntary adoption agency, based in South Street, Reading.
Julie MacLean, its events and support engagement assistant, said the medals were sent as a "small token of thanks to supporters for their time, commitment and fundraising efforts".
"When I posted them, I had imagined them bringing joy to the recipients rather than disappointment," she said.
"I know that one child in particular had been excited to receive their medal so that they could take it to school to show their friends."
A Royal Mail spokesperson said: "We apologise for any inconvenience caused and are actively working to locate [the medals].
"We would advise that customers use padded mailing bags to avoid any posting issues in the future."
