G4S runs both HMP Oakwood, the troubled £150 million jail in Featherstone, South Staffordshire, and HMP Birmingham, in Winson Green.
Today it emerged that the security firm has been fined a total £135,552.57 for not hitting targets set out in its contracts with the Government to run the prisons.
The reasons for penalties have including not hitting targets on mandatory drug testing, the employment of inmates and the number of prisoner complaints. The punishments were revealed after a request under the Freedom of Information Act.
The biggest proportion of the fines was at HMP Birmingham, where the company was made to pay £85,381.93 for not hitting targets over the first three quarters of last year. That was when latest figures were available.
It was fined a total £50,170.64 for not hitting targets at HMP Oakwood, over the first two quarters of last year.
Bosses stressed that fines helped ensure good performance and despite the penalties, public safety had not been put at risk in these instances.
A G4S spokesperson said: "In all prisons (public and private) there will be occasions when elements of the regime are not delivered exactly in accordance with agreed levels.
"This is normal in custodial settings. Within the prison system all privately-run establishments pay financial penalties if the delivery doesn't entirely match the contract.
"While we always strive to deliver all elements in full we accept that we should be penalised if we don't."
A Prison Service spokesman added:
"We work closely with all private sector providers to ensure that the services provided are of the highest standard and that we are getting the best possible value for taxpayers."