Security firm Secro has confirmed it will repay any amount agreed to be owed to the Government.
Serco Group chief executive Christopher Hyman said: "Serco is a business led by our values and built on the strength of our reputation for integrity.
"These values lie at the heart of the many thousands of our people who are endeavouring to deliver the highest standard of service to our customers around the world. We are deeply concerned if we fall short of the standards expected of all of us.
"We are therefore taking this extremely seriously and will continue to work closely with our customer to resolve their concerns in this matter.
"We will not tolerate poor practice and behaviour and wherever it is found we will put it right."
More top news
Charities could face criminal sanctions for bullying the public into giving money, their regulator has warned.
A 'fat tax' could be imposed on the cremation of bodies in Liverpool that require a coffin 30 inches wide or more.
Controversial columnist Katie Hopkins has described her fear that she may die during brain surgery to cure her epilepsy.