Pilots of a passenger heading to land at Heathrow were left stunned when they were involved in a near-miss with a large drone.
Two pilots spotted the gadget, which had up to eight rotors and multiple arms, flying 25 times higher than rules allow, the UK Airprox Board (UKAB) said.
They both exclaimed: "Was that a drone? At 10,000 feet."
The pilots had "no time to react" as they watched the unmanned flying object, just feet away from them, for around five seconds as it passed down the right side of the plane, according to the report.
Police officers at the airport were alerted to the near-miss, but the person operating the drone could not be traced.
The pilots estimated the distance between the aircraft and the drone to be 30 metres (100 feet) vertically and 200 metres (656 feet) horizontally.
The UKAB concluded that "safety was not assured" during the incident on November 11 last year, and determined the risk to be the second most serious category.
The incident was one of four near-misses between aircraft and drones in the latest monthly UKAB report, bringing the total over the past 12 months to 59.
Civil Aviation Authority (CAA), which regulates all aspects of civil aviation, including aviation security, rules state that drones must not be flown above 400 feet or near airports or airfields.
In November the CAA launched a website to publish its revised code of conduct for drones, called the 'dronecode'.
Government ministers are considering mandatory registration for new drones, to crack down on reckless users.
The proposal is part of a Department for Transport consultation on improving drone safety.
It is hoped the scheme could help authorities identify the owners of drones being flown illegally.
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