The police’s decision to launch a surveillance operation against a woman whose brother had been unlawfully killed in a police station was likely to have been influenced partly by racial discrimination, a leaked report by a watchdog concluded.

Janet Alder and her barrister were put under surveillance by police during the inquest into the death of her brother, Christopher. The surveillance team followed them to a hotel and car park and attempted to eavesdrop on their private conversations.

An investigation by the Independent Police Complaints Commission (IPCC), now called the Independent Office for Police Conduct, found there was “evidence to support the conclusion that discrimination and bias is likely to have been a factor” when police initiated the surveillance.

In the unpublished 2015 report, seen by the Guardian, the IPCC said it was more than likely that police stereotyped Alder and her supporters as troublemakers or menacing and treated her barrister, Leslie Thomas, less favourably than his white colleagues when he was followed.

It is the second time the watchdog has made this kind of criticism during the long-running controversy over the death of Alder’s brother, a former Falklands veteran, in 1998. The IPCC suggested in 2006 that police may have been influenced by unwitting racism in the way they treated Christopher before his death – a finding disputed by officers.

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Christopher Alder, who was 37, died handcuffed and face down in a police station in Hull after choking on his own vomit. Officers joked and chatted as he died, and said later they had thought he was play-acting. A tape captured monkey noises as he lay dead.

Recently attention has centred on the surveillance launched against his sister and her barrister during the inquest in 2000, which found that he had been unlawfully killed. This month two police officers were cleared of gross misconduct at a disciplinary hearing.

The IPCC’s report on the surveillance found that Thomas was the only one in the legal team working for the Alder family who was put under surveillance. “There is no indication that any other member of the legal team, who were white, was followed. In these circumstances, it is considered more than likely that the family barrister was treated less favourably than other legal representatives at the inquest.”.

Janet Alder had been campaigning for details about how her brother died. The report said it was “more than likely” that she was probably seen “as stereotypically a troublemaker intent on causing problems for the police by arranging demonstrations and seeking media attention”.

The IPCC cited a memo written by a police employee as another example of possible stereotyping of black people as potential criminals or troublemakers. The employee reported to a senior officer that Alder’s supporters had filled the public gallery at the inquest, stating: “There was a mixture of black and white supporters, but many of the black supporters were large, sported dark glasses and attempted to look menacing.”

The IPCC reported that the memo “made no comparison of how the white supporters were dressed or looked in their appearance; the attendance of black supporters at court appears to be sufficient to anticipate some form of trouble. It is a concern that the report was prepared for one of Humberside’s most senior officers with the use of terminology that is less favourable to black people and intended to influence any subsequent decision-making.”

This month’s misconduct panel heard evidence that the surveillance operation was launched at short notice after police received a report about the presence of “four large black men” who “looked intimidating”.

DCS Judi Heaton, of Humberside police, said: “This report about the events that took place in July 2000 revisits issues that were highlighted at the time and that we have worked hard to address.

“We would like to provide reassurance that we now have a much better understanding of the issues and have implemented an improved programme of training in diversity, as well as making changes to our policies and procedures. This includes identifying the individual needs and risks for every person that we deal with.”