Officers were alerted after employees at a UPS depot in Langenhagen found the item on Tuesday.
Bomb squads and sniffer dogs have been deployed.
Planes that were due to land are currently in a holding pattern.
Passenger Adam Nicholls said: “Just landed back at Heathrow Airport after Hanover Airport got shut down due to a suspect package after we had circled over head for 30 minutes.
“Great start to the trip.”
More to follow…