Widespread disruption to letters and packages being delivered to homes across the whole of the South West is likely after Bristol and Bath depots were evacuated on Friday, September 11.
Police were called to the sorting office on Gloucester Road North shortly after 2am.
It's believed the package may have been transported to the delivery office on Manvers Street in Bath.
A bomb disposal unit was subsequently called to the Bath branch as searches of both depots took place.
People are being warned of possible disruption to their mail being delivered.
A spokeswoman for Royal Mail added: “Bristol Mail Centre and Bath Delivery Office have been evacuated as part of an ongoing police operation.
"Customers in the South West region may experience disruption to services. We are working hard to process and deliver mail where possible."
A nearby depot in Bristol was also evacuated as a result of the incident.
A spokeswoman said: "Police are investigating a bomb threat at the Bristol Mail Centre. This premises is owned by Royal Mail, not Post Office.
“We do have a nearby depot and staff have been asked by police to evacuate the area along with other local businesses.”
