No items of concern were found by officers after a bomb squad was sent to Bristol and Bath Royal Mail depots.
Specialist officers, police dogs and an army bomb disposal unit searched the offices over fears a suspicious package had been sent.
Emergency crews were descended on the Filton premises at around 2:17am (Friday).
Investigations at the Bristol branch led officers to believe the parcel had been moved to Bath.
A cordon was set up around both locations, and a “controlled explosion” was carried out in Bath this afternoon as 'a precautionary measure'.
Officers at Avon and Somerset Police have confirmed this evening 'no items of concern' were found.
A force spokesperson said: "Searches of two Royal Mail sites have concluded with no items of concern found.
"We received a call at 2:17am this morning (Friday, September 11) about a possible suspicious package at the Filton sorting office.
"Officers attended and after speaking with staff, established the package was likely to have already left the site and taken to the delivery office in Bath.
"Searches were subsequently carried out at both sites by specialist officers and dogs while Army's Explosive Ordnance Disposal unit also assisted in the search in Bath.
"A controlled explosion was carried out as a precaution by teh EOD on a package found at the Bath site - the contents of which ultimately proved to be harmless.
"We'd like to thank anyone affected by this incident for their patience while we responded."
Royal Mail has warned residents across the South West delivery of their letters and parcels might be affected.
A spokeswoman for the service said: “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”.
