NATS has seen revenue slip to £886 million for financial 2019, down from £913 million last year.
The figure comes despite an increase in airline traffic.
The UK-based air traffic controller handled a record 2.54 million flights last year, up from 2.52 in 2018.
Net debt also rose, from £72 million to £74 million, while capital investment fell to £157 million, down from £186 million.
NATS chief executive, Martin Rolfe, said: “The group reported a profit before tax which was £34.6 million less than the prior year.
“This mainly reflected real price reductions to our en route customers, and investment in additional staff to meet the growth in air traffic volumes and to progress our technology change and airspace programmes.”
He added: “This was an excellent year for service performance with the delivery of a safe and resilient air traffic control service, every day, against a backdrop of growing traffic and while introducing complex new technology into our terminal control operation.
“We safely handled 2.54 million flights in the year, which was a record volume for the UK, and represented just under 25 per cent of all flights in Europe.
“While doing this, we completed the deployment of electronic flight strips, a major new electronic system which replaced a paper system.
“This followed a complex transition plan that we developed with airlines and airports to minimise the impact on their operations, and we received very positive feedback on the manner and timing of this deployment.
“Our average delay per flight (for the 2018 calendar year) was 12.5 seconds, and included 4.8 seconds of delay specifically associated with this transition.
“In spite of this our performance was five times better than the average delays experienced elsewhere across Europe.”