An immigration centre under scrutiny over the alleged mistreatment of detainees is to be operated by a private company after G4S decided not to renew its contract.
Serco will take over managing Brook House near Gatwick Airport, West Sussex, from May, the Home Office said.
It will also run nearby Tinsley House as part of the £200m deal.
In September 2017, the BBC's Panorama filmed detainees apparently being mistreated.
Serco said it will increase staff numbers to improve the centres.
The Home Office revealed the outsourcing giant had won the contract, despite it being fined £19m last year for overcharging the government to carry out electronic tagging - prompting an audit of its contracts.
A spokesman for the government department said the contract - which will last until 2028 and could be extended to 2030 - was awarded after an "open and rigorous procurement process" and there are "ambitious plans" to improve the two centres.
The Home Office said Serco has promised to:
- Increase staff, including those responsible for welfare
- Give detainees access to educational activities through the week
- Provide new safeguarding and vulnerability training
Serco has been running Yarl's Wood Immigration Removal Centre in Bedfordshire since 2007, as well as five prisons in Britain.
Immigration Minister Kevin Foster said: "Detention plays a key, but limited, role in the wider immigration system.
"This contract is a major step forward in our programme of immigration detention reform. It will significantly improve the day-to-day lives of detainees and the staff who support them."
Following the BBC's investigation, an inquiry into the allegations of mistreatment was announced. It has been delayed pending the legal action.