G4S has agreed to pay back around £24m to the UK government for overbilling on its electronic tagging contracts.
The outsourcing group, along with rival Serco, was alleged to have charged for prisoners who were not tagged or in extreme cases, were actually dead.
After an independent review by Linklaters, G4S said it had wrongly billed for monitoring services in some cases "when equipment had not been fitted or after it had been removed."
The company has also apologised to the Ministry of Justice, but it said the review found there was no evidence of dishonesty or criminal conduct.
It has issued credit notes worth £24.1m, and has paid £2m for the external investigation.
But it warned the amounts may not tally with the Ministry's audit findings.
The contracts run by G4S and Serco are currently under a criminal investigation by the Serious Fraud Office, and both await the outcome of a Cabinet review of their contracts.
Shares in G4S have fallen 0.8p to 256p following the announcement.
Both G4S and Serco have changed their top executives and begun major reorganisations since the problems with government contracts emerged.
Commenting on the Linklater review's findings, G4S new chief executive Ashley Almanza said:
The way in which this contract was managed was not consistent with our values or our approach to dealing with customers. Simply put, it was unacceptable.
As part of a wider programme of corporate renewal, we have changed the leadership of our UK business and we are putting in place enhanced risk management and contract controls.
