Air traffic controllers at National Air Traffic Services (Nats) said there could be long delays for airlines and passengers unless the staffing crisis is resolved.
As controllers work longer hours during systems training, they earn an extra 10 days off. If these days are taken in addition to the normal six weeks' holiday, Nats could be left understaffed, said controllers.
Nats has offered controllers an extra £8,000 if they work on their 10 days off but, as Computer Weekly revealed last week, the proposal was rejected overwhelmingly. Already under pressure in their jobs, controllers say days off are vital and highly valued.
Now controllers are to consider a Nats suggestion that they defer taking their extra days off for three years, by which time there should be a full compliment of staff trained on the new systems.
A reaction to this suggestion is to be put to controllers this weekend at the annual meeting of air traffic controllers from the Institution of Professionals, Managers and Specialists (IPMS).
"There is no easy solution," said a senior member of the IPMS, which represents most air traffic controllers. "People are already working near to or at capacity. They need their days off."
Many large organisations bring in contractors to overcome shortages while employees train on new systems and to cope with the productivity dip that follows the introduction of upgraded technology, but this would be difficult for air traffic control.
Nats has promised to recruit 180 more controllers, instead of the expected 120, but it can take four years to train them. Systems at the New En Route Centre are due to go live in January next year after a six-year delay.
Nats denied that it is facing a staffing crisis. It said there are enough controllers but it would like to have more in reserve in case of high levels of sickness. "We have offered to meet union and staff representatives to discuss their concerns and no date has yet been fixed," said a Nats spokesman.