The chair of the House of Commons Home Affairs Committee, Yvette Cooper, told them it was a matter of "very grave concern" that the company appeared to have failed to get a grip on staff misbehaviour following earlier revelations of mistreatment at a young offenders' unit.
Undercover filming by the BBC's Panorama at the Brook House Immigration Removal Centre, near Gatwick Airport in West Sussex, showed a "system failure" at G4S, she said.
In a bruising hearing at Westminster, G4S regional president for the UK and Ireland Peter Neden assured the committee that action was being taken to prevent a repeat of scenes in which staff were filmed physically and verbally abusing detainees.
Ten members of staff had been suspended following the broadcast, of whom three have been dismissed, and G4S had commissioned an independent inquiry and was assisting a police investigation, he said.
"I was ashamed of what I saw," said Mr Neden. "I am very sorry for what we saw. If we were in any way aware of any of that behaviour we would have taken action.
"We are undertaking an immediate action plan to make sure that this can't happen again. We take these events very seriously indeed. There is no place for behaviour of that kind in our business."
Ms Cooper responded: "But it did happen and you weren't aware of it and you clearly had no systems in place to ensure that if anything like any of these incidents happen they are reported and senior management are aware of it to prevent it happening.
"You clearly have a system failure to allow these things to happen in the first place."
After taking more than an hour of evidence from Mr Neden and G4S's managing director for custodial and detention centres Jerry Petherick, Ms Cooper told them: "The answers that you have given do not suggest you have any grip on this at all.
"None of this suggests you have any idea why this has gone wrong so substantially on your watch - detainees who you had a responsibility towards and yet have been abused while in your custody. You have told us nothing that gives us any confidence that actually have have a strategy for dealing with this or for understanding how it could happen in the first place."
The Labour MP added: "G4S has been through this too many times and been questioned on these issues too many times and for you only to be able to give the same answers you have given before without showing you have a serious grip on dealing with these issues is a matter of very grave concern, given the seriousness of what is happening."
Earlier, the cross-party committee heard evidence from a former G4S duty director at Brook House, who said he had raised concerns about staff and management culture in institutions run by the company between 2001 and his resignation in 2014, speaking to senior management, police, MPs and ministers without sparking any action.
Nathan Ward told the MPs that he was "not surprised but shocked" at the level of abuse revealed in the Panorama film, which included footage of staff apparently mocking detainees who were receiving medical treatment after self-harming or taking the drug spice.
Mr Ward said he had "no faith" in the whistle-blowing systems operated within G4S and told MPs that anyone speaking out faced being "marginalised within the organisation".
Since raising concerns, he said he had received anonymous letters and phone calls and had his car tyres slashed.
Mr Neden denied reports that G4S makes profits of more than 20% on its £11.2 million-a-year Home Office contract for running Brook House and rejected allegations that it charged for staff and activities that did not exist.
But he declined to reveal the true level of profit on the grounds it would put the company at a competitive disadvantage - something which Ms Cooper told him was "not acceptable".
"We are making a profit on these contracts, but we are not making the profits reported," said Mr Neden. "If the Home Office have concerns, the Home Office should raise them."
Mr Ward said it was "categorically" the case that G4S was boosting its profits by charging for staff and equipment which it did not have. In one example of cash-saving by the company, he said G4S had bought clippers to allow detainees to cut each others' hair rather than employing a hairdresser.