Police deploy Taser stun guns too often, with black people more likely to face prolonged use lasting over five seconds, an official report has found.

The study by the Independent Office for Police Conduct (IOPC) warned of a loss of legitimacy after a review of 101 Taser cases from 2015-20. They represent the most serious cases it investigated, and a fraction of more than 94,000 uses over that period – leading police to attack the findings as selective and out of date.

Previous data have shown that black people, who make up less than 4% of the UK population, face greater Taser use, leading to claims of racial bias. The IOPC said that, of the cases it reviewed, 60% of black people who were subject to Taser discharges endured them for more than five seconds, compared with 29% of white people.

It added that mental health played a key role, saying: “In the majority of cases involving either allegations of discrimination or common stereotypes and assumptions, there was evidence that the individual concerned had mental health concerns or a learning disability. This supports findings by others that the intersectionality of race and mental health can increase the risk of higher levels of use of force.”

In one case cited, a black 17-year-old male who was an inpatient at a mental health centre was shot with the electrical weapon three times, struck with a baton more than 20 times and subjected to incapacitant spray and restraint.

The IOPC said that in almost a third of cases studied, chances may have been missed to de-escalate the situation. In a quarter of cases, Tasers were used for compliance despite official guidance to officers that they should not be used in this way. Some 26 investigations out of 101 led to a finding that an officer should face a disciplinary case or be considered for criminal prosecution.

In the last five years, four inquests have found that Taser use contributed to or was one of a combination of factors that led to a person’s death.

In June PC Benjamin Monk was convicted of the manslaughter of the former Premier League footballer Dalian Atkinson in Telford, Shropshire. Monk used a Taser for 33 seconds and kicked Atkinson twice in the head. The victim had underlying health issues, and on the night he clashed with police was facing a mental health crisis that led him to make threats.

The IOPC said: “In incidents where mental health was a factor, people were more likely to be subjected to multiple and prolonged discharges than the overall sample.” It added: “We found examples of good practice where officers recognised signs that an individual may have been experiencing acute behavioural disturbance and responded in line with policy and guidance.”

The watchdog also said in a third of cases it reviewed officers made offensive comments when using Taser but accepted this could happen during stressful situations, saying: “This included officers swearing at, and making derogatory comments to, the individuals, and making unprofessional remarks to them and their families.”

The IOPC director general, Michael Lockwood, demanded reforms, saying: “Ultimately, policing has to change and be more responsive to community concern or risk losing legitimacy in the eyes of the public.

“In particular, people from black, Asian and minority ethnic backgrounds deserve a clear and transparent answer from police on why such disproportionality still exists – failure to address this risks undermining the legitimacy of policing.”

Responding for police chiefs, Lucy D’Orsi, the chief constable of British Transport Police, accused the report of ignoring the realities of policing. “Unfortunately, this report by the IOPC is vague, lacks detail, does not have a substantive evidence base and regrettably ignores extensive pieces of work that are already well under way and, indeed, other areas where improvement could be made,” she said.

“Only 101 Taser uses over a five-year period were reviewed and these were all ones that had been investigated by the IOPC. It is concerning that this only represents 0.1% of all Taser uses in the same period, which totals 94,045.”

Lockwood said the police chiefs’ response to the report was “extremely disappointing”.

More officers are being issued with Tasers, which are more often pointed but not fired, nearly doubling to about 32,000 incidents in the year 2019-20 compared with two years earlier.

British police are largely unarmed and officers see Taser as vital to protect them against a rising tide of injuries and what they say are increasing threats from violent criminals and terrorism.

Amnesty International said: “We need concrete steps to eradicate racist police use of Tasers and to prevent their misuse against vulnerable groups such as children or those with mental health issues.”