A passenger jet taking off from Gatwick Airport was involved in a near miss with a preceding aircraft, according to an official investigation.
The second plane was cleared to climb, bringing it "into conflict" with the first, the UK Airprox Board (UKAB) said.
When the air traffic controller responsible for the flight realised what was happening he "issued avoiding action to the pilot", the report stated.
The incident on April 25 involved two Airbus 319s.
The UKAB found that the near miss was caused by the air traffic controller not scanning his radar display before clearing the second pilot to climb.
"He should have seen (the first aircraft) ahead," the report added.
A "standard procedure" of reducing the usual separation time between departing aircraft to increase frequency - from two minutes to 45 seconds - 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 was reduced to 700ft vertically and around 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.