Deaths in custody should be investigated with the same urgency as murder inquiries, says a long-awaited report that aims to restore trust in the police.

The report, to be unveiled by home secretary Amber Rudd on Monday following a 15-month delay that angered families of those who died in custody, says agencies such as the Independent Police Complaints Commission (IPCC) need to handle deaths following contact with officers with the same haste and mindset as homicides.

Critics say the current approach, where it is not unusual for families to wait for more than five years only to see officers cleared of wrongdoing, can mean that witnesses have trouble recalling events and increases the chance of vital evidence being lost.

The report, which confirms that racial disproportionality exists within the criminal justice system, contains more than 100 recommendations that would profoundly redefine police-community relations if implemented.

Commissioned by Theresa May in July 2015 while she was still home secretary to alleviate the “pain and suffering of families still looking for answers”, the independent report says it is imperative to tackle the drawn-out process facing loved ones seeking justice.

By contrast, murder investigations rely on the “golden hour”, the period immediately following a murder in which securing the testimony of witnesses or the retrieval of evidence such as CCTV footage before it is erased is deemed vital by detectives.

“Speeding up the process is essential to restoring confidence and trust among families,” said a source close to the report, which was overseen by Dame Elish Angiolini QC.

Two recent high-profile cases, both involving the deaths of black men, have been cited as proof by campaigners that the system needs speeding up.

On 4 October three officers were found not guilty of lying under oath over the death of Kingsley Burrell, 29, who died shortly after being detained by police more than six years ago. Days later six officers were cleared of misconduct at a disciplinary hearing following the death of Olaseni “Seni” Lewis in 2010.

One current case concerns Rashan Charles, 20, whose death after being restrained by police in July sparked protests in east London. His family have expressed disappointment with the initial approach by the Metropolitan police and the IPCC.

Rashan’s great-uncle Rod Charles, a former Met chief inspector, recently lamented the “slow, closed and secretive [process of] justice” and the fact that the family were “advised to remain quiet and directed to wait until the end of an investigative process that will not conclude imminently”.

Omar Khan, director of the race equality thinktank Runnymede Trust, said: “It doesn’t send a vote of confidence to families that these sort of delays become routine. Also, the fact that [police] statements look like they have been drafted in such similar ways, or get lost, reformulated or are so out of line with what victims or families say happened.

“Trust and confidence in the police is already low, so there’s a duty to be extra transparent, extra procedurally accountable.”

Other recommendations from the report include creating a closer working relationship between the Crown Prosecution Service and the IPCC .

The campaign group Inquest says more than 1,000 people have died in police custody since 1990, with not a single officer convicted in a criminal court.

The introduction of a national oversight body is also recommended to make sure that coroners’ reports aimed at preventing future deaths are acted upon.

Elsewhere the report calls on the IPCC to challenge racial bias and condemns “victim blaming” through media leaks, a move that creates “false narratives” which deflect scrutiny away from the actions of the police. The family of Rashan Charles were horrified by initial suggestions that he had swallowed drugs, a claim that was later proved to be false.

The report also backs the introduction of body cameras for frontline officers.

Speaking at a rally against custody deaths, held outside 10 Downing Street on Saturday, Ajibola Lewis, the mother of Osaleni, said the seven-year-quest for justice had left wondering what was the point.

“It’s been a waste of time, why bother? But we consulting our lawyers and are not going to give up,” she said.