Ryanair is threatening legal action against Hertz after the car hire company terminated its supplier agreement.
A contract under which Hertz supplied car hire to Ryanair passengers was terminated at midday on Thursday.
As a result, Ryanair says its passenger won’t be able to book car rental through its website for around three months while it seeks a replacement.
It said the disagreement centres on Ryanair selling through distribution companies, which it has been doing since April 2014.
According to the airline, Hertz alleges that Ryanair’s agreement with global distribution companies (GDSs) represents a breach of contract.
But Ryanair disputes the accusation, saying it has been distributing through GDSs for over 15 months with Hertz’s knowledge and support.
"We regret Hertz’s decision to end our car hire agreement at such short notice today, at a time that will cause maximum inconvenience to our customers during the peak travel period," said Ryanair chief Michael O’Leary.
"We have had a long and successful partnership with Hertz, and it’s a pity that this has ended in such an unfortunate and untimely manner."
He said Ryanair has instructed its lawyers to issue proceedings against Hertz for breach of contract.
In the meantime, it is entering into discussions with other car hire suppliers who wish to offer their services.
O’Leary said since Ryanair expects any losses arising from the interruption will be recovered from legal proceedings against Hertz, the airline does not expect it to have a material impact on current earnings.
Michel Taride, group president of Hertz International, said it had enjoyed many years of a ‘successful relationship’ with Ryanair, but said: "Unfortunately we were not able to reach agreement with the airline over a significant contractual dispute in spite of considerable effort.
"Hertz deeply values all of its partnerships, some which have been going for more than 20 years. We strongly believe in mutually beneficial partnerships and would never take lightly the decision to end a key relationship.
"We are confident in our legal position and intend to pursue this matter vigorously through the courts."
Hertz said Ryanair passengers who need its car hire services in July and August are already booked and confirmed, and advised others to make arrangements with Hertz directly.
"Customers can also modify existing bookings on the Ryanair website," added Taride.
The exclusive contract with Hertz was recently extended for a five-year period to 2020.
