A policeman has been captured on tape allegedly assaulting a young black teenager just hours after a colleague of his was recorded abusing another man with a serious racial slur.

PC Joe Harrington has been placed on restricted duties after he was allegedly seen kicking the 15-year-old to the ground and kneeing him, in the custody area of an east London police station. Part of the incident was recorded on CCTV.

An independent investigation into the alleged assault on the 15-year-old concluded last week and a report, understood to recommend disciplinary action, was submitted to the Metropolitan police on Monday. The force will now consider what action if any to take against Harrington.

Hours earlier, Harrington was present when another officer, PC Alex MacFarlane, was recorded on a mobile phone telling Mauro Demetrio, 21, a black man from Beckton in east London, that "the problem with you is you will always be a nigger". Growing controversy over the two incidents, which occurred shortly after the riots last summer, have triggered urgent reviews by the Crown Prosecution Service into initial legal advice that neither officer should be charged.

The audio recording of the racist abuse caused a public outcry after it was released by the Guardian on Friday. It has unnerved senior officers at Scotland Yard, who are concerned about the fallout for the reputation of a force seeking to show it has recovered from recent controversies.

Bernard Hogan-Howe, the Metropolitan police commissioner, is understood to have told colleagues he was appalled when he listened to the recording.

The recording was made by Demetrio, who alleges he was strangled and racially abused after being arrested and placed in the back of a police van on 11 August – the day after the end of rioting in London last summer.

The mobile phone recording captured one officer saying that he strangled him because he was "a cunt". Moments later, PC MacFarlane abuses Demetrio and adds: "You'll always have black skin colour".

Harrington was not heard making any racist remarks on the recording, but was one of three officers initially investigated over the alleged mistreatment of Demetrio. MacFarlane, who can also be heard telling Demetrio "don't hide behind your black skin", has been suspended.

A CPS lawyer was asked to review the case in January, after an Independent Police Complaints Commission (IPCC) investigation concluded that three officers – MacFarlane, Harrington and one other officer – may have committed criminal offences.

Carl Kelvin, from the CPS police complaints department, initially decided no charges should be brought against any of the police officers.

On Thursday, after being threatened with legal action by Demetrio, the CPS said it would ask a more senior lawyer to reconsider the initial decision, which was partly based on a belief that no alarm or distress was caused by the racist abuse.

Demetrio has spoken about how the racist abuse he suffered has left him feeling traumatised. Shortly after being driven to Forest Gate police station, he told police in the custody suite that he had been abused by police officers and urged them to listen to his mobile phone.

It was while he in the custody area that Demetrio witnessed Harrington allegedly assault the 15-year-old, who was handcuffed. Demetrio told investigators he saw Harrington kick the young teenager in the back of the leg and, once he was on the floor, knee him in the back.

He said the alleged assault made an "echoing" sound and the teenager cried out: "I am on the floor now – you can't do anything to me. I am handcuffed and I am on the floor."

Demetrio said that medical staff were called to the scene after the teenager, whose identity is not known, began making "strange" breathing noises for several minutes.

After Demetrio reported what he had seen, a separate IPCC investigation was launched into the case of the 15-year-old and CCTV of the incident was obtained – although the quality of the footage has been described as poor.

At an early stage of the investigation, IPCC investigators sought advice from the CPS to establish whether Harrington had committed a criminal offence.

"The CPS advised that on this occasion there was not a realistic prospect of a conviction in relation to the criminal offence of common assault," said IPCC Commissioner Mike Franklin.

The CPS lawyer who gave that advice was the same lawyer whose decision not to bring charges against officers recorded racially abusing Demetrio is now under review.

"This is clearly an important matter and I have this evening directed that urgent inquiries be made about any advice that may have been given," said Grace Ononiwu, deputy chief crown prosecutor for CPS London. "Once the full facts are known I intend to issue a further statement."

Keith Vaz MP, chair of the Commons home affairs select committee, wrote to the director of public prosecutions on Monday asking for detailed information about the decision-making process in the Demetrio case.

"You have on a number of occasions made it very clear that the CPS would act swiftly to resolve cases when it is in the public interest to do so," Vaz wrote to Keir Starmer. "I consider that this is such a case."