By Ian Shoesmith and Tom Symonds
Air traffic controllers are under so much pressure to maximise the number of landings that passengers' safety is at risk, the BBC has been told.
Britain's air traffic control system denies any pressure on controllers
A retired air traffic controller, who asked not to be named, said an "errant culture" at Heathrow focused on service provision, more than safety.
He described one incident over Reading last year as the most risky he had seen in 27 years of air traffic control.
Britain's air traffic control system, Nats, said safety was the top priority.
The whistleblower, who has spent more than half his 27 years' service working on the Heathrow operation, is well known to National Air Traffic Services (Nats).
In the months before he retired in February, Nats asked him to write a report examining potentially dangerous safety breaches on the approach into Heathrow.
He showed the BBC a copy of the 33-page confidential document, which included detailed analyses of 12 incidents in the year to October 2007.
The report was acknowledged, in writing, by his manager as being "very accurate".
Heathrow's runways are currently handling 98% of their total capacity, and the controller wrote in his report: "A premium on optimum air traffic control performance ... has encouraged controllers to implement inappropriate plans of action and continue with them after it has become clear that (they) cannot be satisfactorily executed".
He later told the BBC: "In some areas we are throwing away some of the safeguards because it is the easiest way of moving the aeroplanes. I believe you call that cutting corners."
'Turned in time'
He points to one incident over Reading on 28 September, 2007.
The official Nats investigation report, also obtained by the BBC, concluded that an air traffic controller deliberately turned one plane directly into the path of another at the same altitude.
By the time evasive action took effect, the aircraft were just 1.8 nautical miles and 100 feet vertically apart. The legal minimum in this situation is 2.5 miles.
The BBC source said the incident involved a "greater risk than I have ever seen in 27 years of air traffic control".
"The controller quite deliberately, although not maliciously, put aeroplanes in a 'fail-dangerous' situation, in order that he could maintain the runway service rate. And in so doing he endangered the travelling public," he said.
A senior Nats manager privately acknowledged the incident was an example of extremely poor air traffic control technique, but publicly Nats insisted this and the other incidents were not serious because aircraft were turned in good time.
None were classified as a near-miss.
Alex Bristol, general manager of the Swanwick Nats unit in Hampshire, which is responsible for flights approaching Heathrow, insisted safety was the "primary concern, at all times".
"We have hugely skilled controllers who have been trained over many years. We have systems and procedures in place which ensure safety.
"We have a reporting system and culture which I believe to be second-to-none in Europe, certainly, and probably in the world."
Controllers have a system which alerts them to potential collisions, and airliners have one which tells pilots what avoiding action to take.
Mr Bristol strongly denied claims controllers were being put under undue pressure to maximise the flow of aircraft.
The BBC source expressed further concerns about the spacing of aircraft as they come in to land at Heathrow.
A minimum distance, typically of three miles, must legally apply between pairs of aircraft.
However, when a large plane like a Boeing 777 is in front, a bigger gap must be left behind as larger planes create heavy turbulence in their wake.
Nats' own figures suggest minimum spacing is regularly being breached on the final approach - typically 20% of flight arrivals each month.
But the source stressed no individual controllers at Heathrow should be blamed for that.
"I believe the culture has become errant and I believe that is a danger to the public," he said.
Training and advice
He said Heathrow controllers' "try and put [aircraft] as close together as we really dare within the limits of the law.
"That is the only way we can actually service Heathrow at the [required] demand," he said.
He believes safety standards could be improved, without sacrificing capacity, by introducing rules to guarantee aircraft are at different heights when they turn to make their final approaches.
But Nats managers told the BBC this would be a major change to the air traffic control system, affecting approaches to other airports aside from Heathrow.
Nats admitted there had been instances of poor technique in the handling of Heathrow-bound aircraft, and the Civil Aviation Authority confirmed there had been an increase in incidents.
Nats says controllers have now received extra training and advice.