The British government has called in fraud investigators after auditors found security giant G4S overcharged by millions of pounds on contracts to monitor convicts using electronic tags.

Justice Secretary Chris Grayling said Thursday he was asking the Serious Fraud Office to investigate after G4S refused to take part in an audit of its contract.

He said "the billing practices in question were clearly unacceptable."

It is the latest bad news for G4S. Last year, Britain had to call in thousands of troops to help with security at the London Olympics after G4S acknowledged it couldn't provide the 10,400 guards it had been contracted to deliver.

This week, prosecutors said they were considering criminal charges over an Angolan man who died after being restrained by G4S guards during deportation from Britain.