Two passenger jets came within 700 feet (213 metres) of each other after a mistake by a British air traffic controller, according to a near-miss report.
The error was made by a controller "during a period of high workload", the UK Airprox Board (UKAB) said.
He gave an instruction to increase altitude to a Boeing 737 leaving Stansted Airport, Essex, which was meant for another aircraft from the same airline, the UKAB said.
This caused the former to climb "into confliction" with a Boeing 777 approaching the airport, according to the report.
The official minimum separation between the two jets should have been 1,000 feet (305 metres).
After realising his mistake the controller gave further instructions, but the pilot of the Boeing 737 claimed these were "very unclear" and he "sensed some confusion from the controller".
The pilot described the risk of collision as "medium-high".
The incident occurred at 6.57pm on April 1.
Some members of the UKAB suggested the controller may have been "overloaded".
The report concluded: "The board quickly agreed that the airprox (near miss) had occurred because the controller had inadvertently instructed the wrong aircraft, B737, to climb into confliction with the B777.
"In mitigation, it was apparent to the board that, at the time of the airprox, the controller's workload was high."