An aircraft taking off from Gatwick was involved in a near miss with another up ahead, according to an official investigation.
The incident on April 25 involved two Airbus A319s taking off from the airport. The airlines the aircraft belonged to were not identified.
The UK Airprox Board (UKAB) found the cause was the air traffic controller’s failure to scan the radar before clearing the second pilot to climb.
“He should have seen [the first aircraft] ahead,” the report said.
When the air traffic controller responsible for the flight realised what was happening he “issued avoiding action to the pilot”.
A “standard procedure” of reducing the usual separation time between departing aircraft from two minutes to 45 seconds to increase frequency was given as a contributory factor.
The UKAB added that poor co-ordination by air traffic control staff also had an impact.
Separation between the aircraft, which can carry up to 156 passengers, was reduced to 700ft vertically and about two miles horizontally.
The report found that safety had been “degraded” but the rate of catch-up between the two aircraft was very low.
The incident was assessed as having the third most serious degree of risk by the UKAB, the BBC reported