A Belgian trade union warned on Tuesday that Ryanair is "playing with fire" in its handling of a planned cabin crew strike across five countries on September 28, by not warning passengers about possible disruptions to flight schedules.

The company is continuing to sell tickets for flights on Friday and is not responding to queries from customers about the status of their flights on that day, the Belgian cabin crew union CNE said in a statement.

"This situation is totally irresponsible," the union said. Ryanair did not immediately respond to a request for comment.

Cabin crew unions in Belgium, Spain, Portugal, Italy and the Netherlands are planning a 24-hour strike on September 28 over Ryanair's labour agreements. They plan to strike once a month until their demands are met.

Some pilots' associations are also planning to join the strike.

The Belgian cabin crew union predicted that the strike would be the biggest ever seen in Europe and cause the cancellation of about 80 per cent of flights from Charleroi Airport and probably all of the airline's flights from Brussels Airport.

Earlier in September, Ryanair rejected claims by the unions that the strike would cause "travel chaos" and said the airline would warn affected customers of a small number of flight cancellations. 

The strike would be the biggest ever seen in Europe and cause the cancellation of about 80 per cent of flights from Charleroi Airport and probably all of the airline's flights from Brussels Airport

During a walkout by pilots and crew earlier this month, the airline was forced to cancel 150 of its 400 German flights.

Customers on Twitter have asked the Irish airline about the status of their flights on September 28.

Ryanair has responded to some that it expects "the vast majority of cabin crew to work as normal on September 28, as they have during previous strikes" and that "all fights are scheduled to operate."