Police officers involved in the stopping, searching and handcuffing of the British sprinter Bianca Williams and her partner are now under investigation for gross misconduct over alleged racism and dishonesty, the Guardian has learned. Williams and Ricardo dos Santos were stopped on 4 July last year in north-west London by officers from the Metropolitan police’s Territorial Support Group. They were searched on suspicion of having drugs and weapons, with none found, while their three-month-old son was in the back seat. The investigation by the Independent Office for Police Conduct began into lower-level misconduct claims against five officers. But in an intensification of the proceedings, the formal investigation has now become more serious after new evidence was unearthed by investigators. Three Met officers have been notified by the IOPC they are under investigation for gross misconduct. If proven, the maximum penalty is dismissal. All of these three officers are being investigated for whether their actions were influenced by racial bias. Two of those are also being investigated for whether they acted dishonestly. The police basis for searching the two elite athletes was that officers believed they could smell cannabis. Three other officers are under investigation for misconduct offences, one of whom is facing an allegation they may have acted in a discriminatory way. The issuing of discipline notices does not mean charges will follow and all officers are understood to deny wrongdoing. Both athletes are trained by the former Olympic champion Linford Christie, who accused police of institutional racism. Video of the incident showed Williams distressed, and telling officers: “My son is in the car.” The case was one of a series of videos surfacing last summer on social media that raised concerns about alleged discrimination, police tactics and policies. The Met denies that it is discriminatory and says it is reviewing the use of handcuffs on those stopped and searched. The Met police commissioner, Cressida Dick, backed her officers and tried to quell the furore by saying that “any officer worth their salt would have stopped that car”. Williams is a gold medal winner at the Commonwealth Games, and her partner, Dos Santos, is a Portuguese sprinter. The reason police gave for stopping the vehicle was that it was driving erratically. Dos Santos was driving the Mercedes in west London about 1.20pm. Williams was in the rear seat with their baby. The incident lasted an hour and the couple alleged a baton was raised by officers, some of whom were aggressive, with Dos Santos claiming he was “dragged” out of the car. Investigators have reviewed video from the police vehicle, body camera footage from the officers and data from both vehicles. The athletes, both preparing for the Tokyo Olympics, believed the initial IOPC investigation was too weak and did not take their claims of racism seriously enough. Ricardo dos Santos told the Guardian: “Finally this at least looks like a more credible investigation now. “The officer dragged me out of the car with a raised baton and handcuffed me, he falsely alleged that he could smell cannabis on me and recorded this as the justification for the search on the stop and search form. “Discrimination and a lack of honesty was at the heart of the officers’ behaviour that day and now at last it is what they are actually being investigated for.” Race is an issue that haunts the Met and the couple faced abuse on social media after the incident. They were also angered by the Met dismissal of their complaints and rapid defence of their actions. Williams said she lacked confidence in the current commissioner: “There is a problem of racism in the Metropolitan police, but it is not going to go away unless there is a commitment from the top to tackle it. It felt like there was a campaign led by the police commissioner to discredit us and shut down our complaint. I am not hopeful that issues of racism can be addressed in the force under this commissioner.” Their solicitor, Jules Carey, said: “The IOPC have taken a year to upgrade the investigation to an investigation in racism and dishonesty. I have huge admiration for my client’s resilience and patience.” The Met said all officer remained on normal duties: “We are aware three MPS officers are subject to a gross misconduct investigation by the IOPC in relation to this matter. Another three officers are being investigated for misconduct for potential breaches of standards of professional standards. The MPS continues to fully cooperate with the IOPC investigation. “No officer is suspended or subject to restricted duties.” The IOPC explained why it had upgraded its investigation to one of gross misconduct against three officers: “After reviewing a range of new evidence, they were informed they are now subject to an investigation for potential breaches of the police standards of professional behaviour relating to equality and diversity, which requires officers to act with fairness and impartiality and not to discriminate unlawfully or unfairly. “Two of those officers are also now being investigated for potential breaches of the standards relating to honesty and integrity, requiring them to be honest and act with integrity at all times. “Collectively, these alleged breaches amount to potential gross misconduct.”