More than 100,000 items of mail have been delayed in Devon due to a bomb hoax at a city sorting office.
A suspect package was discovered at the main Exeter sorting office by a member of staff at midnight on Thursday.
Bomb disposal officers sealed off the area for about four hours while inspecting what police have called an "elaborate" device featuring a concoction of cola bottle lids, powder and wires put into the post with no address.
Police are warning whoever was responsible faces a jail sentence of up to seven years under the Terrorism Act if they are caught.
Forensic examination
Extra staff have been brought in to try to deal with the backlog. Royal Mail says the hoax has caused major disruption.
Graham Jones of Royal Mail said: "All our mail which normally comes in through Exeter Airport couldn't get into the centre for processing.
"Most of that we have been unable to get out to delivery offices, so mail which customers were expecting on their mats will not be there until Friday."
Pc Roy Adams of Devon and Cornwall Police said: "Under the Terrorism Act of 2002, it is actually an offence to send an item like this through the post. We're taking this very seriously."
The fake bomb is being sent off to forensic scientists who will test it for fingerprints and DNA.