Human rights groups have accused the Government of holding back a report into deaths in custody, which was due to be published a year ago.
Theresa May commissioned an independent review on police custody deaths two years ago when she was home secretary.
It was due to be published in the summer of 2016 but has not yet been made public and there is currently no release date.
A group of 30 charities, community groups and a retired police chief superintendent have signed a letter calling on Home Secretary Amber Rudd to release the findings immediately.
The statement, published in the Guardian, said: "The continued and unexplained delays suggest the Government is holding back on publishing the report.
"We therefore call on Home Secretary Amber Rudd to immediately release the report of the independent review into deaths in police custody.
"Families have been calling for transparency and justice on deaths in custody for decades.
"By releasing the report of the independent review the Government can begin to convince bereaved families that it is committed to transparency and justice for the families affected by deaths in custody."
The letter cites the case of Sarah Reed, 32, who was found dead in her cell at HMP Holloway in north London in January 2016.
She was previously assaulted and struck in the head by a police officer while being held on suspicion of shoplifting.
"Her relatives will be expecting answers on the level of care she received and her wider treatment by public authorities," it added.
Scottish lawyer Dame Elish Angiolini was appointed to conduct the review in 2015.
It was set to examine the procedures and processes surrounding deaths and serious incidents in police custody, considering the extent to which ethnicity is a factor in such incidents.
Mrs May previously said that deaths in custody have the "potential to undermine the relationship between the public and the police" and that she was "struck by the pain and suffering of families still looking for answers".
The Home Office said the review "will be published in due course".