During late 2000 and 2001, Nats gave repeated assurances that there were enough controllers to handle normal air traffic control operations and to train on new systems at the Swanwick New En Route Centre.
In November 2000, Nats defended allegations by air traffic control unions of staff shortages by telling the House of Commons Transport Committee that it had built up access to about 60 extra controllers to handle the transition to Swanwick.
The training of air traffic controllers on the new systems continued through most of 2001. The Swanwick control centre went live in January 2002.
Now a leaked memo to staff written last July by senior Nats executive Ian Hall, the then general manager at the London Area and Terminal Control Centre at West Drayton, has revealed that Nats was at times "very short of controllers".
The letter admitted, "Day-to-day shortages of air traffic controllers have been causing huge delays."
Hall added, "Many of you [controllers] have found it confusing at least, offensive at most, that we have stated publicly that we are not short of air traffic controllers whilst sectors [areas of air space] have been under-staffed."
Although Nats had more than the 334 controllers it needed, Hall's letter said that sickness and the wrong mix of skills, exacerbated by training at Swanwick, had "frustrated the calculations so that we have clearly been very short of controllers at times".
In his memo, Hall said that in one week in 2001 delays to airlines had been twice their normal level and "over half the 180,000 minutes of delay were attributable to staff shortages".
This week a spokesman for Nats denied that the public and MPs had been misled. "We stand by what Ian [Hall] said at that time," he said. The spokesman insisted that last year there had been "no overall shortage of controllers".