The envelope aroused suspicious and emergency services were called after it was feared the item could contain a hazardous substance.
It was discovered in the post room at 10am and after concerns were raised about it, the police and fire services were alerted.
Anxious staff sent tweets saying they couldn’t leave the building.
It is understood a white powder was found in the envelope but this was found to be harmless.
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One fire engine and crew from South Shore were called to the council buildings in Corporation Street, Blackpool, at 10.24am, while a specialist detection, indentification and monitoring (DIM) vehicle in Manchester was put on standby.
A Lancashire Fire and Rescue spokesman said white powder had been found in the post, sparking the lock-down.
“We were there working with the police to test the substance just to check what it was,” he said.
Though he was unable to say what the powder was, he said it was ‘non-hazardous’.
When asked who the package was addressed to, a council spokesman declined to comment, citing the ongoing police investigation.
The council spokesman added: “An incident at Blackpool Municipal Building was reported this morning to the emergency services.
“Police officers, Lancashire Fire and Rescue, the Ambulance service and Blackpool Council senior staff attended the site following receipt of a suspicious package.
“We were advised that the contents were non-hazardous with no injuries to our staff reported.
“An investigation is now being undertaken by the police and we will provide every assistance to them.”
A Lancashire Police source said: “There were concerns that there may be a harmful suspect and precautions were needed while there was a potential risk.”
The lock down ended at around 12.30pm.