G4S overcharged the UK government by £24.1m on a contract to carry out the electronic tagging of criminals (Reuters)

An internal review at security giant G4S has found that it did overcharge British taxpayers on an electronic tagging contract with the Ministry of Justice by billing for work it had not carried out.

As a result of the review, carried out by law firm Linklaters, G4S said it will pay back £24.1m to the MoJ for incorrect billings between 2005 and 2013 on tagging of criminals it did not carry out. The firm has been accused of charging the MoJ for the tagging of dead or still-incarcerated prisoners.

The Serious Fraud Office (SFO) was conducting a criminal investigation into G4S as well as its rival Serco which is accused of similar practices, justice secretary Chris Grayling has said.

G4S said that the Linklaters review, which trawled internal documents and emails, had "not identified any evidence of dishonesty or criminal conduct by any employee of G4S". The investigation cost G4S £2m.

"Our announcement follows months of intensive work by an independent law firm and external accountants and is an important step in setting this matter straight and restoring the trust which has been earned by our many thousands of committed and hard-working employees over many years," said Ashley Almanza, G4S group chief executive.

"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 and we have apologised to the Ministry of Justice.

"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."

He added: "We remain committed to working with the ministry and the government to resolve this matter and to provide enhanced oversight of service delivery and contract performance."

In October, Richard Morris, G4S regional chief executive of UK and Ireland, resigned. Morris had been at G4S for more than a decade and, before becoming UK boss, was managing director of electronic monitoring at G4S Care & Justice Services. The firm would not give details on why Morris quit.

"This matter is now the subject of a criminal investigation.We are not able to comment further at the current time.We will make a further statement when it is appropriate to do so," said a spokesman for the MoJ.