Cathay Pacific has placed warning signs inside lavatories on its Airbus fleet to prevent passengers from clogging toilets with bulky items, an airline spokeswoman confirmed Wednesday.

The move comes after a spate of mysterious blockages on Cathay planes, including one last month that forced a Riyadh-Hong Kong flight to make an unscheduled landing in Mumbai, causing an 18-hour delay.

Two other Airbus flights - one from Rome on November 9 and another from Dubai on November 19 - had to restrict passenger numbers when it was discovered that the toilets onboard had malfunctioned.

The problem could be due to passengers flushing away bulky items, the carrier has said.

The new stickers on the inner and outer lid of toilets show pictures of towels, cups, containers, napkins, sick bags and other items beneath a sign in English and Chinese warning: "These items will choke the toilet. Please dispose of them in the waste bin."

Cloth towels in first-class lavatories will be replaced with paper towels until the end of the month.

Cathay Chief Executive Tony Tyler sent an internal staff memo describing the grounded flights as "very unfortunate," the South China Morning Post reported.

The carrier's engineers are now fitting new pipes and carrying out deep-cleaning treatment on toilets, the paper added.

Aircraft toilets use pipes that carry waste up to 110 kilometres an hour (69 miles per hour) into a holding tank that is emptied between flights.

Vacuum systems operate separately down each side of the plane, meaning a blockage usually affects all toilets on one side of the aircraft.

The carrier's internal regulations say that planes must have at least one toilet per 80 passengers, meaning passenger numbers are restricted if some lavatories are out of service.