The "suspicious package" discovered at the Royal Mail sorting office in Hanwell was not harmful, police have confirmed.
Royal Mail said it was in fact a leaking powder which police later confirmed as a bleaching agent.
Metropolitan Police specialist officers who examined the contents of the package found it to be "non-noxious."
All staff have since returned back to work following the scare on Thursday morning (March 15).
Three people were treated at the scene as a precaution but thankfully did not have to be taken to hospital.
A Royal Mail spokesman said: "We can confirm that a parcel leaking powder was discovered in our Hanwell Delivery Office this morning (Thursday March 15).
"The incident was reported to the police and the area was cleared as a precaution.
"The police confirmed that the contents of the parcel was a bleaching agent and the item was not sent with any malicious intent.
"No staff were injured and all are back at work, with mail deliveries being made to customers in Hanwell as usual.
"The safety of our people is of the utmost importance to Royal Mail and we have strict procedures in place to deal with any such incidents, which fortunately are very rare."
Emergency services including the ambulance and police were sent to the scene of the incident.
Road closures were put in place while the incident was dealt with.
A Metropolitan Police spokesman said: "Police were called at 7.41am on Thursday (March 15) to reports of a suspicious package at Hanwell Sorting Office, in Edinburgh Road.
"Specialist officers attended the scene and the content of the package was assessed.
"The contents have been found to be non-noxious.
"London Ambulance Service attended and three people were assessed as a precaution.
"The incident has been stood down."
A London Ambulance Service spokesman said: "We were called at 7:56am today to Edinburgh Road, W7, to reports of an incident.
"We sent an incident response officer and an ambulance crew. We also dispatched our hazardous area response team (HART).
"We assessed several people at the scene but have not taken any to hospital."
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