The first near miss between passenger plane and a drone has been investigated by air safety inspectors.
The UK Airprox Board will publish its findings on Friday following the incident at Heathrow Airport.
The owner of the drone has still not been identified raising fears over the potential risks drones could pose to commercial airlines .
The incident is expected to be recorded in the report with a risk rating A, defined as a serious risk of collision, which is the highest of five categories.
The report, seen by the The Sunday Times, said the pilot of the Airbus A320 spotted the drone, which failed to show up on air traffic control systems, at 2.16pm on July 22 while flying at an altitude of 700ft.
Earlier this year the airline pilots' association Balpa demanded better protection for the public against the risks of drones.
It wants drones, officially known as Remotely Piloted Aircraft Systems (RPAS), which share airspace with passenger and freight airliners, to meet the same safety standards as piloted aircraft.
This would include only being flown by operators with pilot-equivalent training.