The Irish carrier urged passengers to head to airports anyway, saying it had asked pilots to show up for duty and pledging to do its “upmost to minimize any disruptions”.
Germany's powerful Cockpit union said pilots would hold a four-hour “warning strike” at German airports from 5-9 am (0400-0800 GMT) after initial talks with the airline were cancelled at short notice.
“All pilots directly employed by Ryanair will be called to strike,” Cockpit said, as national news agency DPA estimated the strike would affect some 16 flights with a total of 3,000 passengers.
If the call is heeded, it will mark the first industrial action by Ryanair pilots in the company's 32-year history.
Ryanair said it “sincerely regrets” the move, calling the industrial action “unjustified and unnecessary” as it had assured the union earlier in the day to continue talks on a collective labour agreement.
The airline said it “apologizes sincerely to any German customers worried or affected by this threatened four hour strike” but urged them to stick with their travel plans.
“We advise all customers in Germany to turn up as normal tomorrow, as we plan to operate all scheduled flights, and we will be doing our upmost to minimize any disruptions to the Christmas travel plans of our German customers,” said a statement by the company's Robin Kiely.
The union Cockpit (VC) said Ryanair broke off the first scheduled talks this week because it objected to two of the five union members at the table, which it charged proved the company had “no desire to enter into constructive negotiations”.
It accused Ryanair of playing for time to avoid upheaval over the hectic Christmas and New Year period.
“Ryanair's public offer to conduct negotiations with VC can only be classified as a further publicity stunt,” said Ingolf Schumacher, head of Cockpit's industrial department.
Ryanair last week took the unprecedented step of offering to finally recognize unions after crew in Germany, Ireland, Britain, Italy, Spain and Portugal threatened walkouts in long-running rows over pay and conditions.
The move eased tensions but unions warned that strikes remained an option if the Dublin-based carrier was not serious about the discussions.
Just hours before the German strike call, Ryanair was able to stave off year-end travel chaos on home soil after clinching a deal with Ireland's Impact union.
The union there said the danger of industrial action had “receded for the present” after Ryanair agreed to formally recognise Impact as the representative for the airline's pilots.
But it cautioned that it expected management to reach agreement on procedures quickly so that the parties could move on to negotiate “substantial issues” around pilots' pay and working conditions.
In Germany, Cockpit said it regretted the travel disruptions and urged passengers to contact Ryanair about the impact on flights.
“In the history of the VC, there has never been a case in which the collective bargaining autonomy has been trampled on by an employer as it is now the case with Ryanair,” it added.
Ryanair's decision to move towards trade union recognition marks a historic turning point, given that pugnacious boss Michael O'Leary – in charge since 1994 – had vehemently opposed any union representation for staff.
But he came under increasing pressure after the airline was forced to cancel 20,000 flights through to March because of botched holiday scheduling.
The fiasco triggered pilots' demands for better working conditions and representation, with some departing for other carriers.
Ryanair's conditional offer to recognize pilot unions prompted unions in other European countries to suspend their strike plans.
The Italian union Anpac said it planned to meet with Ryanair representatives in Rome in January.
Ryanair, Europe's second-largest airline by passenger numbers, has set itself the goal of transporting 200 million passengers annually by 2024.
Despite the recent troubles, it still expects to deliver annual profits after tax of €1.40 billion-€1.45 billion ($1.65 billion-1.71 billion).