The Czech army said Tuesday anti-missile systems for its four CASA C-295M military transport aircraft had failed testing and would require repairs before being deployed in missions abroad.
"The (DAS anti-missile protection) system has failed and requires adjustments," chief-of-staff spokeswoman Jana Ruzickova said, adding it failed to work in "seven out of 17 areas tested".
The defence ministry was in talks with the Spanish producer on a repair, she said.
"These are not system faults, but defects must be removed to make the system fully functional," she said, adding the trouble prevented the deployment of the aircraft abroad.
The army purchased the planes in 2010 to replace its obsolete Soviet-made Antonov An-26s, but the new aircraft have since been grounded several times due to defects.
Repairs on the avionics and communication systems allowed training and transport flights, but deployment in missions has been deemed unsafe.
Experts and media alike have also slammed the purchase as overpriced, suggesting corruption may have played a role.
In June, police requested parliament to strip former defence minister and now lawmaker Vlasta Parkanova of immunity pending her criminal prosecution over the purchase.