The airliner was heading for Florence from London when a passenger - a journalist with the Daily Telegraph - noticed the problem from his window seat.
A section of the wing detached and became loose before the plane turned back to the airport.
Passengers said they heard a loud bang and saw a part of the wing hanging on.
Journalist Cole Moreton said: "On take-off there was a strange noise then a bang and six-foot plus section of the left wing that looked like suddenly came loose and hung down, flapping in the jetstream.
“It seemed hydraulic pistons had snapped as they were flapping too. Passengers were alarmed and told the cabin staff, who appeared to inform the captain."
The Avro RJ85 jet circled back on itself and landed at London City Airport before being met by emergency services on the runway, with all 61 passengers on board unharmed.
A CityJet spokesman said: "The WX 281 flight had just taken off from London City Airport on its way to Florence when the cover of the operating mechanism on the wing became partially detached.
"The crew followed their standard procedures and, after a discussion with CityJet Maintenance Control, decided to return to London City Airport.
"Air Traffic Control were notified and the aircraft returned making a normal approach and landing. Passengers are being re-directed on another aircraft and will depart with a two hour delay."