Thousands of Medibank Private customers will likely have to delay filing tax returns because of major technical blunder.

Medibank revealed on Thursday that the majority of its 3.8 million customers would probably not receive their tax statements by July 15 because a technology upgrade had gone awry.

Medibank customers will likely have to delay filing tax returns. Credit:Glenn Hunt

Taxpayers earning over certain thresholds need to include statements from their health insurer with their tax returns to avoid paying the Medicare levy surcharge.

Medibank CEO Craig Drummond said the situation was unacceptable, but assured customers their personal details had not been compromised.