The home secretary has been accused of sitting on a report from a critical inquiry into deaths in police custody.

A coalition of community and human rights groups want Amber Rudd to announce when the report will be published and say why it has been delayed.

The report was due to be completed last summer. The Home Office has not given a date for its release.

The review was set up by Theresa May while she was home secretary and announced after the Conservative election victory of 2015. May believed that the criminal justice system made it too hard for people whose loved ones had died in police custody to get answers. The review was also meant to examine whether ethnic minority people suffered more.

In a letter published in the Guardian, the coalition of groups – including Liberty, the Runnymede Trust and criminal justice groups – say: “Two years ago Theresa May, as home secretary, commissioned an independent review on police custody deaths, including investigating racial disproportionality.

“The review’s report was due for publication over a year ago. The continued and unexplained delays suggest the government is holding back on publishing the report.”

The letter adds: “By releasing the report of the independent review, the government can begin to convince bereaved families that is committed to transparency and justice for the families affected by deaths in custody.”

The issue of deaths after police contact continues to test relations between the police and communities they serve. Last Sunday there were disturbances in east London after Edir Frederico Da Costa died following contact with police. An investigation is under way into the circumstances of his death.

Monday is the ninth anniversary of the death of Habib Ullah after he was stopped and searched by officers from Thames Valley police.

Omar Khan, of the Runnymede Trust, a race relations thinktank, said: “The unexplained and long delay in publishing this independent review reflects poorly on the government’s willingness to deal directly and transparently with deaths in custody, and risks more people losing confidence in the criminal justice system. There have been many controversial deaths in the past 30 years, with families feeling that deaths were preventable.”

Before ordering the review, May met the families of Sean Rigg and Olaseni Lewis, who died after restraint by officers in 2008 and 2010 respectively.

The review was meant to be a foundation for a landmark reform of people’s experiences of the police. She hoped it would end “evasiveness and obstruction” suffered by families at the hands of the authorities and “transform the relationship between the public and the police and to preserve the historic principle that the public are the police and the police are the public”.

The government appointed Dame Elish Angiolini, formerly Scotland’s top prosecutor, to conduct the review. She was helped by Deborah Coles, director of Inquest, which campaigns on behalf of people who have lost loved ones after contact with the state, and also Raju Bhatt, one of the leading solicitors in holding police to account in the courts.

Leroy Logan, a former Met officer who also signed the letter, said: “The longer they sit on it the more it builds people’s suspicions. The last thing anyone needs, when there are so many issues with trust and confidence in the state, is a review of this importance being withheld.”

Martha Spurrier, director of Liberty, said: “Too many people die in custody and too many deaths in custody are preventable. Without proper systems in place to learn the lessons of these wasted lives the state is failing to honour their memory, listen to their families and make custody a safer place.”

The Home Office said: “The review will be published in due course.”

It announced on Friday that the report had been delivered to the home secretary.