Six Metropolitan police officers accused of responding to an offer of assistance from an off-duty black firefighter by abusing him, dragging him from his car and shooting him with a Taser should face disciplinary charges for possible racial discrimination, the official watchdog has concluded.

Edric Kennedy-Macfoy has accused police of behaving like wild animals when he approached them in a north London suburb to provide them with a description of a man he spotted throwing a rock at a police van.

After a 20-month investigation into the case, which involved tracking down members of the public who witnessed the incident, the Independent Police Complaints Commission (IPCC) concluded there was evidence the officers racially stereotyped the fireman, according to a summary of its key findings in the case, which has been seen by the Guardian.

The IPCC is referring a dossier of evidence to the Crown Prosecution Service after concluding a police constable could also face criminal charges over the use of the Taser. The watchdog also found police failed to act with integrity, courtesy, patience, discretion, professional judgment or common sense.

The most senior-ranking officer among those accused, former inspector David Burgum, denied the charges, questioned the firefighter’s motives and took the unusual step of condemning the statutory watchdog that investigates serious police complaints.

“In my opinion Mr Kennedy-Macfoy has cynically played the race card for his own ends,” Burgum said in a statement to the Guardian. “I do not consider that the IPCC have conducted an independent investigation. They are political organisation with a strong anti-police bias.”

Kennedy-Macfoy was driving through Harrow around 3.30am in September 2011 when he saw a young man hurl the rock at the police van. After noting a description of the young man, Kennedy-Macfoy flagged down the van driver and approached a line of officers to pass the information on.

A disagreement ensued in which, the IPCC said, several officers used abusive language against Kennedy-Macfoy.

The off-duty fireman complained officers repeatedly swore at him, before charging at his car and pulling him from the vehicle.

In an account he gave the Guardian in 2012, Kennedy-Macfoy, then 29, said he responded by calmly and showed his palms to the officers, telling them: “Listen guys, I haven’t done anything wrong. I’m a firefighter – I work with you lot and I just want to explain something.”

He said the Taser was discharged, without warning, when he was walking backwards with his hands in the air.

The final report from the IPCC investigation has not been made public or provided to any of the parties involved.

However, a summary of the IPCC’s key findings, seen by the Guardian, concludes the police’s initial reaction to Kennedy-Macfoy was based purely on his ethnic appearance.

The watchdog’s report names six officers, including Burgum, who it says have a case to answer for gross misconduct in respect of their alleged racial discrimination of Kennedy-Macfoy.

In addition to the ex-inspector, they include a sergeant, three police constables and a special constable. One of the constables - the officer who twice discharged the Taser - could also face criminal charges, the IPCC states.

“The IPCC has completed its investigation into a complaint made by Edric Kennedy- Macfoy relating to his arrest by [Met] officers in September 2011,” a spokesperson for the watchdog said. “The IPCC will be referring a file of evidence to the Crown Prosecution Service for consideration related to an officer’s use of Taser.”

The IPCC did not find sufficient evidence to uphold a complaint against two officers: a seventh officer, of constable rank, who was also present on the night, and a detective inspector who initially handled the fireman’s complaint.

After Kennedy-Macfoy was shot with the Taser, he was arrested and charged with obstructing police. He was found not guilty after a trial at Brent magistrates court.

During those proceedings Burgum gave evidence about the fireman’s racial appearance, which later formed part his complaint. Burgum told the court his officers were in a “stressful” situation and had been dealing with a group of partygoers who had been throwing missiles at them.

According to a court clerk’s notes of proceedings, Burgum added: “I couldn’t say he was anything to do with the party. The party was all black. He was black. He had driven through the cordon. I had to do a quick risk assessment.”

Burgum retired from the Met in January and now works for a private company that has an outsourced contract to train prospective Met police recruits. In his statement to the Guardian, Burgum called the account given by the fireman and apparently supported by the IPCC investigation “implausible in the extreme”.

The ex-inspector is among the four officers the IPCC concluded have additional cases to answer for misconduct on the night – in his case, for swearing at Kennedy-Macfoy.

Burgum said it was ridiculous for the IPCC to raise concern about his abusive language toward fireman because “Mr Kennedy-Macfoy swore at me first”.

“The suggestion that the police reaction to Mr Kennedy-Macfoy was based purely on his ethnic appearance and that the police officers racially stereotyped him is likewise ridiculous,” he said, adding that some of his police colleagues present on the night were “of ethnic minority backgrounds” and they, too, reject the suggestion that this was “a racial incident”.

Given he has retired, Burgum cannot face disciplinary proceedings. However, the other five officers, who all remain at the Met, could be subject to a misconduct hearing. The Met declined to say whether it would hold such a hearing.

“As is normal procedure, we will consider the report’s finding and associated evidence and respond to the IPCC within the statutory 15 working days,” a Met spokesman said.

If the Met decides against holding the hearing, the IPCC has powers to compel the force to do so. Asked if the watchdog planned to use that authority, an IPCC spokesperson said: “We’ll cross that bridge if we come to it.”

Kennedy-Macfoy’s solicitor, Shamik Dutta of the firm Bhatt Murphy, said: “In light of the IPCC’s findings we now look to the CPS and the Met police commissioner to properly consider all the evidence that has been gathered and to make a decision which does justice to that evidence.”

The IPCC and Met have clashed over the Kennedy-Macfoy case before.

His complaint was initially investigated by the Met’s professional standards department, with arm’s-length supervision from the IPCC. That internal inquiry provisionally concluded no police officer should face disciplinary or criminal proceedings.

The IPCC then took the unusual step of rejecting the Met’s inquiry in its entirety, initiating the fully independent investigation. It is that inquiry that, after almost two years collecting and analysing the evidence, concluded six officers have a case to answer for racially-motivated misconduct.