The carrier had already axed 146 out of some 2,400 scheduled flights across Europe because of strikes planned by Irish, Belgian and Swedish pilots on Friday.
Speaking at a press conference in Frankfurt, Ryanair's chief marketing officer Kenny Jacobs said the airline regretted the “unnecessary strike action”.
Germany's powerful Vereinigung Cockpit union has called for a strike among all Ryanair captains and pilots stationed in Germany this Friday.
Cockpit said it had asked its 400 Ryanair pilots to walk off the job for 24 hours from 03:01am (01:01 GMT) on Friday, affecting all of the airline's outbound flights.
“We are extremely sorry for the affected passengers. The responsibility lies with Ryanair management,” Cockpit president Martin Locher told a press conference on Wednesday.
The action will take place in tandem with strikes in Sweden, Ireland and Belgium, as Ryanair continues to struggle with a summer-long wave of industrial action across Europe.
The dispute began with several, weekly 24-hour strikes from pilots in Ireland earlier this summer, and they were joined in July by Ryanair employees in Belgium, Spain, Portugal and Italy.
Europe's second biggest airline has been grappling with staff unrest since it recognised trade unions for the first time in December 2017, in a bid to ward off widespread strikes over the Christmas period.
But unions say their calls for better wages and fairer contracts have gone unheard, and no progress has been made despite months of negotiations.
Two weeks ago, members of the Vereinigung Cockpit voted by 96 percent for similar strike action among pilots stationed in Germany.
Cockpit said that said Ryanair management had failed to respond to a Tuesday deadline for an improved offer, leaving them with no choice to go on strike.
Friday’s strikes will see more passengers hit by cancellations, with more than 100,000 estimated to have already been affected this summer.
“Passengers whose Ryanair flights are cancelled may have a claim to damages of up to €600 per person, providing the flight is cancelled within 14 days of its scheduled departure,” said Laura Kauczynski, an expert on passenger rights at the website AirHelp.
“The same applies for passengers whose flights are delayed by more than three hours.”