The British AirProximity Board released their final report concluding the probable cause of the loss of separation was:
The pilots of aircraft on converging tracks flew into conflict because, although they acknowledged timely avoiding action, they did not follow it.
The British AirProximity Board annotated that the tracks of both aircraft did not appear to have changed significantly until being clear of conflict again. In post occurrence interviews the crew of BA-87 recollected that they were given an immediate right hand turn to 050 degrees to avoid other traffic, while the crew of LH-418 recollected they were given an immediate left hand turn to 270 degrees.
The Board analysed that the controller issued the climb instruction to LH-418 at a point, where there was little distance to go until conflict. However, the controller recognized the pending conflict in time and issued instructions in time that were suitable to maintain required separation. Simulation showed that had the instructions been followed the separation would not have been lost.
The Board continued analysis: "It was apparent that both crews had taken each others’ instructions, and the Board found it hard to determine why this had occurred; unfortunately no Human Factor report was available from either crew. The Board was surprised that all four pilots had misheard or misinterpreted the avoiding action instructions despite at least one of the crews reading them back correctly." A callsign confusion could be ruled out from ATC recordings, all callsigns in instructions and readbacks were clear.
The Board further analysed: "It was possible that the crews may have been distracted because this would have been about the time that they would have been receiving their Oceanic clearances on data-link. Another possibility mooted by an airline-pilot Member was that, having settled into their trans-Atlantic routine, it was unusual for pilots to be issued with avoiding action instructions at that altitude and location. Expecting only routine information to be transmitted at that time, they may have been perplexed by the avoiding action information and instinctively responded without properly assimilating it. He also noted that, during simulation training, avoiding action was only practiced as a result of a TCAS alerts, and not as a result of ATC instructions; this was an important consideration as to their potential familiarity with receiving, assimilating and actioning such RT instructions in a timely manner."
The Board annotated that they were "disappointed" that BA-87 had not reported their TCAS resolution descent.