G4S made profits of more than £2m per year for running an immigration centre where detainees were filmed being mistreated by staff, a report says.
The National Audit Office found the security firm made £14.3m profit running Brook House from 2012 to 2018.
The findings raise serious questions about the Home Office's handling of sensitive contracts, MPs said.
The Home Office said it had been working to improve leadership, management and training at the centre.
Panorama footage broadcast in September 2017 showed alleged assaults, humiliation and verbal abuse of detainees by officers at the centre, near Gatwick Airport.
In total, 21 members of staff were identified as part of the Panorama allegations - 12 were later dismissed and three resigned.
The NAO report found G4S has been making "significant profits" on the Brook House contract.
Between 2012 and 2018, G4S made £14.3m gross profits, (before deducting a share of company overheads, such as human resources), with gross profit rates of between 10% and 20% each year.
It also pointed to the fact that under the terms of its contract G4S cannot be penalised if staff use excessive force or inappropriate language.
Labour MP Yvette Cooper, chairwoman of the Home Affairs Select Committee, said the findings raised "serious questions" about the Home Office's handling of sensitive contracts and claimed the department's monitoring should have picked up problems sooner.
She said: "For G4S to be making up to 20% gross profits on the Brook House contract at the same time as such awful abuse by staff against detainees was taking place is extremely troubling.
"Given that profits reduced when G4S had to increase staffing and training after the Panorama programme, this raises very serious questions about G4S's running of the centre to make higher profits whilst not having proper staffing, training and safeguarding systems in place."
She confirmed that the committee will further pursue both G4S and the Home Office.
Despite the problems, the NAO found that G4S "broadly delivered" on the terms of the contract.
A Home Office spokesman said it was making "significant changes" to the contracting model.
They said: "The Home Office and G4S have been working together and we remain committed to improving leadership, management and training at Brook House."
John Whitwam, managing director of G4S custodial and detention services, said: "Building on the significant progress already made at Brook House IRC, we continue to work closely with the Home Office to improve further the services we provide."
According to the NAO, the Home Office has now concluded that the Brook House contract as written is no longer fit for purpose, given the lack of scope to impose financial penalties and enforce improvements in conditions and treatment.
Any new contract is expected to include new performance measures covering staff recruitment, induction, training, mentoring and culture, and establish a contractual role for the Home Office to monitor the appropriateness of the use of force against detainees and the care of staff and detainees following an incident.