A second jury trying a policeman charged with racially abusing a suspect has been discharged after failing to reach a verdict.
PC Alex MacFarlane, 53, had been accused of telling a young black man under arrest: "The problem with you is that you will always be a nigger".
The first trial took place last week but the jury could not reach a verdict and there was a similar outcome for a second rapid retrial held this week.
MacFarlane is unlikely to face another trial, but there remains the possibility of internal disciplinary action from the Metropolitan police.
During the trial the force said it would examine evidence about MacFarlane gathered by the Independent Police Complaints Commission, but could only do so after criminal proceedings were completed to avoid prejudicing a trial.
MacFarlane, a Metropolitan police officer based at Forest Gate in east London, had been charged with a racially aggravated public order offence against 21-year-old Mauro Demetrio in the back of a police van in August last year.
The Crown Prosecution Service had initially declined to charge MacFarlane, only doing so when Demetrio's lawyers threatened to legally challenge the decision. Further pressure was placed on the CPS when the Guardian ran a story based on audio recordings from inside the van made by Demetrio on his mobile phone, the key evidence in the case.
One recorded passage has MacFarlane using the word "nigger", a comment made in a low voice and seemingly close to Demetrio. The audio records the young man's protest, while MacFarlane goes on to say: "You will always have black skin colour", "don't hide behind your colour", tells him to "be proud" of his race.
An earlier recording made by Demetrio features another policeman in the van calling him a "scumbag" and a "cunt" and boasting about having throttled him. Demetrio complained he had been assaulted, and a police doctor found marks on his neck but no charges were brought.
Duncan Atkinson, prosecuting, told the initial trial at Southwark crown court that MacFarlane's comments were intended to insult and "put Mr Demetrio in his place". He said: "It is clear that this abuse was racially motivated, and was targeted and was intended. Such words were designed to cause - and did cause - distress and insult. They were designed to suggest to Mr Demetrio that he was inferior to the officer because of the colour of his skin."
MacFarlane, an officer for 18 years with a previously unblemished professional record, argued that he was seeking to "defuse the situation" as Demetrio had become agitated and abusive when the officers stopped the car he was driving and arrested him over two outstanding warrants.
MacFarlane and other officers in the van at the time testified that Demetrio had himself been racially abusive, calling police "white cunts" and making sexually explicit threats. None of these alleged comments were on the recordings.
The constable said Demetrio first used "nigger" in reference to himself, in asking why he had been arrested while his white friend was not. Repeating it was an error caused in part by fatigue from long hours over the preceding days, when he had been involved in policing the riots in London at the time, MacFarlane told the first trial.
In his first testimony from the witness stand, MacFarlane argued that his words were a misguided attempt to turn the young man's life around.
"I had formed an impression in my mind that he had low self-esteem," MacFarlane said. "I wanted him to reconsider his lifestyle, to not view his skin colour as the reason behind the problems he had, not to blame the police, not to blame other people."