On the eve of the latest strike by Ryanair pilots employed in Ireland, tens of thousands more travellers face finding their flights cancelled by industrial action on Friday 10 August – one of the busiest days of the summer.

The airline is telling passengers: “Ryanair has received notice of strikes on Friday 10 August by pilot unions in Belgium and Sweden.

“We have also been notified of possible strike actions by pilot unions in Germany and the Netherlands which we believe will be coordinated and occur on Friday 10 August.”

Twenty Ryanair flights between Dublin and airports in Britain have already been cancelled on Friday 3 August, as a result of the fourth one-day strike by pilots based in Ireland. They are involved in a dispute with airline management over seniority and base transfers.

Around 3,500 passengers have been notified that their flights have been cancelled, and offered a choice of full refunds or alternative departures.

The strike the following week is likely to affect far more people, with Irish pilots employed by Ryanair having also given notice of a fifth one-day strike on the same day.

Members of the Irish Airline Pilots’ Association (IALPA) say industrial action will continue “until there is substantial movement on the pilots’ reasonable demands for an agreement on a fair and transparent approach to base transfers and related matters”.

In a statement, IALPA said: “The union has today told Ryanair that it remains available for talks.

“The airline’s escalation of the dispute last Wednesday – when it threatened to sack 100 pilots and 200 cabin crew, or transfer them to Poland – led to a predictable hardening of resolve among its staff.”

Ryanair’s chief marketing officer, Kenny Jacobs, blamed pilots from a rival airline for stirring up trouble, saying: “In most unions where four strikes have failed to have any effect, the unions and employer would be entering into negotiations to resolve the dispute.

“In this case, after four days of unsuccessful strikes, a handful of Aer Lingus pilots are working behind the scenes with pilot unions in Sweden, Holland, Belgium, and Germany to further disrupt Ryanair’s business.”

Ryanair has asked unions to give a week’s notice of strikes “so that we can notify our customers and offer them alternative flights or refunds”.

Finding alternative flights will be a challenge, because the airline is flying its aircraft fuller than ever this summer, with a 97 per cent load factor – in other words, only five or six seats are spare on the average flight.

Ryanair says: “We have written to each of these pilot unions today and invited them to meet us in the coming days to make further progress in concluding Union Recognition and Collective Labour Agreements in each of these markets.”

The airline has recognised trade unions only since December last year. Last week Ryanair cabin crew in Spain, Portugal and Belgium stopped work for two days, leading to the cancellation of 600 flights and scuppering the travel plans of 100,000 passengers.

The UK Civil Aviation Authority has urged passengers whose Ryanair flights are cancelled by strikes to claim compensation of €250 (for journeys of up to 1,500km) or €400 (for longer journeys).