An unpublished Government report is said to have found evidence of "disproportionate" deaths of black and minority ethnic people while being restrained by police.
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 still no release date.
According to the Guardian, the report will say: "There is evidence of disproportionate deaths of black and minority ethnic people in restraint related deaths."
The newspaper said the report calls for radical reform of restraint by police, and makes more than 100 recommendations including an end to police officers conferring after incidents and before they make their statements.
It will also reportedly recommend installing video cameras in every police van used to transport a prisoner and on every frontline officer.
The newspaper says another recommended reform is the police watchdog robustly challenging discrimination, where there is clear evidence and "where it can be inferred", as well as recommending the introduction of mental health training across all 44 forces in England and Wales.
According to the Guardian, the report will say police cells should be completely phased out as a place to hold people believed to have mental health problems.
It will also say that police must be held to account at "an individual and corporate level" if restraint of a suspect is excessive, the newspaper reported.
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 has previously said 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".
A Home Office spokeswoman said they would not comment on an unpublished report, adding that it will be "published in due course".