Durham Tees Valley Airport has been urged to make efforts to ensure passengers are aware of a new departure levy before they arrive for flights.
Adult passengers are now charged a £6 airport facility fee and children £2.
Owner Peel Airports said the move was necessary to safeguard its future in the face of rising costs.
But the Civil Aviation Authority (CAA) has told airport bosses they have an obligation to make passengers aware of the fee before they turn up.
Peel Airport chief executive Craig Richmond said the airport was operating at a loss.
Passengers pay the new fee at machines in the terminal which issue tickets enabling them to pass through security and into departure areas.
Airport staff faced some criticism from passengers who were apparently ignorant of the new charge when it was introduced on 15 November.
A CAA spokesperson said: "The Civil Aviation Authority have contacted Durham Tees Valley about the introduction of this charge to remind them of their obligation to work with airlines to ensure passengers are aware of the fee before they buy their ticket.
"If people arrive at the airport genuinely unaware of the fee, then we encourage them to retain the information about their booking and, if they are unsure of their rights, contact the Air Transport Users Council in the first place for advice.
"The CAA is an enforcer of consumer protection regulations relating to these type of charges."
Mr Richmond said: "It is not something we want to do - this is a great regional airport with a lot of support - but it is a tough time and when you see your passengers drop off by between 30 and 40%, you have to do something to stabilise the finances."