A check on the Ryanair website by Sky News found at least 50 flights listed as cancelled between destinations in the affected countries and the UK.
Ryanair confirmed the cancellations and strike today (Friday August 10), calling it "regrettable and unjustified", as pilots based in Germany, Belgium, Sweden, Ireland and the Netherlands walked out in a dispute over working conditions and pay.
Yesterday, at the eleventh hour, they were joined by Dutch pilots after an unsuccessful attempt by the airline to obtain a court order blocking the strike action. 'The strike may go ahead, ' a judge said. The majority of customers affected have already been re-accommodated on another Ryanair flight.
A Ryanair spokesman said: 'Ryanair took every step to minimise the disruption and we notified our customers as early as possible advising them of their free move, refund or reroute options.
It already suffered a round of strikes by cockpit and cabin crew last month that disrupted 600 flights in Belgium, Ireland, Italy, Portugal and Spain, affecting 100,000 travellers.
Ryanair operates more than 2,000 flights a day, serving 223 airports across 37 countries in Europe and North Africa, and insists it will not change the low-priced model that transformed the industry and has made it Europe's most profitable airline.
Kenny Jacobs, Ryanair marketing director, called the strike "useless", and said that Ryanair pilots are paid better than their counterparts at competitor airlines Easyjet or Norwegian.
Staff are holding a 24-hour walkout over pay and conditions.
Another key complaint of workers based in countries other than Ireland is the fact that Ryanair employs them under Irish legislation, arguing most of its employees work on board Irish planes. This can mean staff based in other European states are unable to gain access to state benefits.
Ryanair has described the German strike as "unjustified".
Ryanair has repeatedly said it remained open to further talks with pilot representatives.
The unrest is one of the biggest challenges to face long-term chief executive Michael O'Leary, who was once quoted as saying he would rather cut off his hand than recognise unions and on another occasion crossed a picket line of baggage handlers to help load a plane.
Shares in the airline were 1.4 percent lower at 13.36 euros by 0825 GMT, having fallen 18 percent since the action ramped up in mid-July to stand well below the 14.21 euros they slumped to in December when Ryanair shocked markets by recognising unions.