Ryanair it to take legal action against Hertz after the car rental firm today severed their long-running partnership.
The Irish carrier said Hertz terminated its exclusive care hire supply agreement with Ryanair at midday.
Ryanair said in a statement: "Hertz allege that Ryanair's agreement with GDS distribution companies represents a breach of contract, a claim which Ryanair disputes given that Ryanair has been distributing through GDSs since April 2014 (a period of over 15 months) with Hertz's knowledge and support."
All existing bookings will be honoured and customers will still be able to modify these bookings online.
However, Ryanair said the two firms had recently extended their partnership until 2020 and that it will now "pursue Hertz for breach of contract and damages".
Ryanair CEO Michael O'Leary said: "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.
"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...
"We apologise sincerely to our customers for the absence of any car hire services on the Ryanair website from 1200 today, however this decision has regrettably been forced upon us by Hertz’s short notice this morning."
Earlier this week, it emerged that Ryanair is set to launch flights to Amsterdam this winter (see news, June 30).