Earlier this week, the Civil Aviation Authority announced that from next month aircraft approaching Heathrow and Gatwick airports would be allowed to stay two-and-a-half nautical miles apart, instead of the current three nautical miles. This will allow an extra four landings an hour at Heathrow.
The trials had been kept under wraps by the CAA, but were uncovered by a German pilot trying to land at Heathrow earlier this year.
Writing in Vereinigung Cockpit Info, a magazine produced by the German pilots' association, the Lufthansa captain describes a "potentially unsafe situation which I would not again tolerate". He said that airport authorities waived the minimum distance between aircraft and then slowed down the jets - placing the two aircraft closer together.
The distance between jets is a safety consideration designed to ensure that vortices, the spinning streams of air that trail behind aircraft, dissipate before other flights follow. The force of vortices can be so great that it produces abrupt movements of an aircraft's nose and inexplicable sharp, steep bankings.
One of the aircraft affected by the CAA ruling is the Boeing 737, the world's most popular passenger jet. It has been involved in 55 incidents of "unusual attitude" over the past five years in Britain - most of which were caused by wake vortices.
The CAA says that airlines were informed of the trials, and co-operated. But Mike Burlyn, of the Guild of Air Traffic Control Officers, said: "It is totally unacceptable to do things this way.
"This is due to become a permanent procedure, but although some airlines were probably aware of the trials pilots themselves were not aware that they were participating. It was never announced through the usual channels. As a result we have no information about any safety analysis of the trials, so we can't make any objective judgement."
A spokesman for the CAA said: "It is a completely safe procedure, and it will only be used infrequently when conditions are perfect."