One of Europe's biggest discount airlines, Ryanair, will charge passengers to use on-board toilets.

The Irish airline is planning to make its toilets coin-operated, forcing passengers to fork out £1 (NZ$2.16) or €1 ($1.89) every time they want to spend a penny in the sky.

The extra charges, which come on top of separate fees for check-in baggage and snacks, will coincide with the reduction of some toilet facilities on Ryanair planes.

"By charging for the toilets we are hoping to change passenger behaviour so that they use the bathroom before or after the flight," Ryanair spokesman Stephen McNamara told the Daily Mail.

"That will enable us to remove two out of three of the toilets and make way for at least six extra seats."

The news came as the carrier came under fire for raising fees for check-in baggage by 33 per cent to £20 a bag.

Rochelle Turner, head of research for British consumer magazine Which? Holiday, said Ryanair was penalising families going on holiday.

"Flying with Ryanair is not always as cheap as it first appears and we would advise passengers to factor in these extra costs when choosing who to fly with," she said.