The pursuit of a senior officer under paedophile laws when she is not a sex offender is discriminatory and an example of institutional racism still poisoning the police force, the Black Police Association has claimed.

The comment came after Supt Robyn Williams, a decorated senior police officer convicted last week of possessing a child abuse video, was sentenced to do unpaid work and put on a register meant for sex offenders.

She was sent the unsolicited video in February 2018 by her sister who wanted the paedophile behind it hunted down and caught.

The judge sentenced her accepting she had no sexual interest in children and said he had to improvise a sentence because of the “very unusual circumstances” of the case.

Williams, one of the most senior female African-Caribbean officers in Britain, was praised for her work after the Grenfell fire disaster. Despite her conviction last week, the public gallery at the Old Bailey in central London was packed with people who came to show their support for her.

She was sentenced to 200 hours of unpaid work with her job after 36 years of “stellar” service. Her lawyers are considering an appeal.

Her sister, Jennifer Hodge, who suffers from anxiety and depression, had been outraged the video was circulating on social media and sent it to all 17 people in her WhatsApp contacts list. Williams claimed she was unaware her sister had sent her the video.

The common serjeant of London, Judge Richard Marks, said Williams’ claims of ignorance were “fanciful”, especially given Hodge was extremely upset by the video and the two sisters had spent hours with each other without Williams reporting it.

The judge told her: “It is a complete tragedy you find yourself in the position you now do.”

Williams was acquitted last week of another charge of corruptly failing in her duty to report the video because she wanted to avoid getting her sister in trouble.

The jury convicted on five of the six counts brought against Williams, her sister and her sister’s partner.

The treatment of Williams, 54, has caused concern among some in policing in the Police Superintendents’ Association, among Williams’ staff and the Black Police Association.

For black officers, the concerns boiled over publicly on Tuesday as Williams, one of the BPA founding members who was awarded honours for her work, was facing what the judge called “immense consequences” for her career.

In a statement, the Met branch of the BPA said the Met had the discretion not to pursue Williams, but had chosen to do so.

“The MetBPA believes that the internal guidance on this matter allows for any person in innocent possession of any image of this kind to be treated with discretion, especially when the investigation has concluded that there has been no sexual gratification. The question that is now asked, is why was this discretion not afforded to Supt Williams.

“The MetBPA position is that this is a classic example of institutional racism. The clip was sent to 17 people and only three people were convicted. It is well documented about the disproportionally that exists within misconduct and complaint of officers and staff from an African, Caribbean or Asian background.”

The video came to light when some of those who received it reported it to police.

The jury acquitted Williams of corruptly failing in her duty, but convicted her of possession.

In his sentencing remarks, the judge gave an interpretation of the verdict. Williams had been sent the video unsolicited by her sister on a Saturday morning. Williams said she had become aware only the following Monday morning when telephoned by her sister, who had by then been arrested by police.

The judge said the jury had rejected Williams’ account and that by 7pm on the Saturday, eight hours after receiving the video, she was aware a video was on her phone and of “its broad context”, given her sister’s extreme reaction to it.

The judge said it was “a grave error of judgment” by Williams to do nothing about the video, adding that, because of her experience as a police officer, she knew the “imperative” to act to safeguard the five-year-old child shown in the video being abused by an adult.

Janet Hills, chair of the BPA, said: “There have been many examples in the last couple of years of black senior officers being held to account at a higher level than their white counterparts. This has to stop. The finer detail of this case needs to be scrutinised to ensure that there is fairness for all.”

The judge said as he sentenced Williams: “The fact that you did nothing about it was a grave error of judgment on your part, especially given the fact that by dint of your job, you knew the imperative of so doing and had the ready means at your disposal to act … It is though an error of judgment from which you neither gained nor stood to gain in any way which makes it all the harder to understand.”

The Met deputy assistant commissioner Matthew Horne said: “The prosecution called this a ‘sad case’ and referred to the ‘serious errors of judgement’ made by those involved.

“The Independent Office for Police Conduct is carrying out an independent misconduct investigation into the actions of Supt Williams and we await the outcome.”

The court heard that Hodge, who was convicted of distributing the image, had lost her job with the charity Scope after 17 years and it had destroyed her relationship with her sister.

She was sentenced to carry out 100 hours of unpaid work. “I have no doubt you are a thoroughly decent woman,” the judge told her. He accepted she sent the video in outrage and the court heard she had wanted those behind it caught and jailed.

Throughout the three-week trial, Williams stayed away from Hodge and her partner, and did not seem to exchange a word or glance.

Hodge’s partner, Dido Massivi, has lost his job as a bus driver after 20 years. The court heard he blamed himself for the damage inflicted on Williams and her career, and his partner.

Massivi, 61, was sentenced to 18 months’ imprisonment suspended for two years, and to carry out 200 hours of unpaid work. He was convicted of distributing two indecent images and possessing an extreme image.