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The Hovding's airbag-style helmet inflates when it detects an accident.
Royal Mail has been criticised for disposing of a special cycling helmet because it "breached safety rules".
Anne Nagle, from Shrewsbury, bought the £323 helmet - which inflates on impact - online but posted it back because it was uncomfortable.
But the teacher received an email from Royal Mail saying her package "had been disposed of" because a gas cylinder inside made it a fire risk.
Mrs Nagle said she should have been contacted before any action was taken.
'Prohibited items'
"What surprised me is Royal Mail opened the parcel and then destroyed it," said the mother-of-three.
"My address was on the parcel, it was a regular parcel. I thought it would have been reasonable... to contact me and say you shouldn't have posted this."
Tony Marsh, the Royal Mail's director of group security, said: "Compressed gas cylinders - whatever the contents - are prohibited items under dangerous goods regulations.
"Many of the compressed gasses can be flammable so that represents a fire as well as an explosion risk."
The Hovding's airbag-style helmet is worn as a neck collar but inflates when it detects an accident.