Two G4S ambulance crew members have been suspended after leaving a voicemail message for a vulnerable patient in which they suggested they could “beat the fuck” out of him and then claim self-defence.

The driver of the two-man non-emergency ambulance crew could be heard telling his colleague that “beating a patient to a pulp” was preferable to “being beaten to a pulp”, even if they were prosecuted. His colleague suggested that to do so would simply result in a six-month suspension while they were investigated.

The patient, who does not want to be named, told the Guardian he was sent the ambulance on 8 May to take him for an MRI scan on his lower abdomen after a stomach rupture. He has mental health problems and had been rehoused in temporary emergency council accommodation in Kent the previous week.

The driver of the G4S vehicle left a message saying the crew would be a few minutes late to take him to William Harvey hospital in Ashford, Kent.

“He’s homeless, he’s aggressive, he’s suicidal, so he ain’t gonna be the best, is he?” the driver can be heard to say. “You don’t need to take him if you don’t feel safe with him. That’s why we’ve got the fire extinguisher. Take it off the hook now and you’ve got something to hit him with. You’d rather be prosecuted, wouldn’t you, for beating a patient to a pulp than that patient beating you to a pulp, wouldn’t you, as a choice of the two?”

After his colleague agrees, the driver continues: “D’you want a broken arm, a broken jaw, black eyes, or do you want to beat the fuck out of someone and then get prosecuted? You could still say it was self-defence.” His colleague then adds: “And get suspended for six months while they check up on it.”

The driver concludes: “[At] least you’re able to sit in your garden. You wouldn’t be able to get into your garden with a broken leg, would you?”

At the end of the four-minute message, the driver realises they are still being recorded. “That was a long message on his phone,” he says, without appearing to register the significance of what the two have said.

The patient, aged 43, had not been picked up by the two crew members before and has no history of being aggressive towards G4S staff. He said the message left him distraught.

“When I got in the ambulance they were all nicey-nice and said ‘how are you mate?’ and I said ‘I was good till I heard your voice message’,” he said. “Then I played them the voicemail. They tried to apologise and I said it’s a bit late for that.” He said he self-harmed as a result of the incident.

On 10 May, G4S wrote to the patient saying: “I am sorry to learn that you had a poor experience with our service.” He was subsequently visited by G4S management, which apologised to him.

He added: “What made it worse is that I was told the two men were suspended on full pay.”

The patient said he was told by Sheerness police that they would not pursue charges because he had not been threatened directly by the G4S crew.

G4S provides patient transport for NHS commissioning groups across Kent and Medway. This is one of six contracts with NHS trusts it has in London and the south-east.

The recording represents the latest blow for the security firm, which has been hit by a series of scandals in the past. It was stripped of its contract to run Birmingham prison last year after inspectors discovered “appalling” squalor in the jail and said violent prisoners were able to act with “near impunity.”

In 2017, an undercover Panorama investigation found detainees were being abused and humiliated at the immigration removal centre Brook House. In 2016 G4S was stripped of its contract to run Medway secure training centre, and the Guardian and Panorama revealed that children were being unlawfully restrained.

In 2013 a G4S security guard, Clive Carter, was jailed for 20 years after bludgeoning to death Khanokporn Satjawat with a fire extinguisher at a conference in Glasgow.

The patient said: “When I read about the woman who had been killed with the fire extinguisher, I thought: this could have been me.”

Politicians criticised the company. Dr Dan Poulter, a Conservative MP and former health minister who is also a part-time mental health doctor, said the incident called “into question the wider employment practices used by G4S”. He said that given its track record, “the suitability of G4S as a provider of important public services in the NHS and elsewhere needs to be urgently reviewed”.

Norman Lamb, a Liberal Democrat former health minister, said: “This recording is deeply shocking. This company is a serial offender. It raises the question as to whether it can be trusted to deliver humane, decent and effective services to vulnerable people. This cannot simply be passed off as a rogue incident … We cannot tolerate these repeated failures by G4S.”

The managing director of G4S patient transport services, Russell Hobbs, said: “We are appalled by these comments. We have apologised unreservedly to the patient, and the employees concerned have been suspended pending a full internal investigation. The comments are completely unacceptable and do not represent our values or the views of the rest of our hardworking team.”

G4S refused to say whether the two employees were suspended on full pay.

Stuart Jeffery, the deputy managing director at Medway Clinical Commissioning Group (CCG), which commissions patient transport services in Kent and Medway, said: “This is a completely unacceptable incident but we welcome G4S’s swift action in suspending the employees involved and launching a full investigation. Although we will be monitoring the situation closely with G4S, we believe this is a performance issue with specific individuals rather than with the service as a whole.”