Almost 100 prisoners have died in US custody in Iraq and Afghanistan since August 2002, according to US group Human Rights First.
The report says that charges for mistreating prisoners are rare
The details were first aired on BBC television's Newsnight programme.
Of the 98 deaths, at least 34 were suspected or confirmed homicides, the programme said.
The Pentagon told Newsnight it had not seen the report but took allegations of maltreatment "very seriously" and would prosecute if necessary.
The report, which is to be published on Wednesday, draws on information from Pentagon and other official US sources.
Human Rights First representative Deborah Pearlstein told Newsnight she was "extremely comfortable" that the information was reliable.
The report defines the 34 cases classified as homicides as "caused by intentional or reckless behaviour".
It says another 11 cases have been deemed suspicious and that between eight and 12 prisoners were tortured to death.
But despite this, charges are rare and sentences are light, the report says.
Speaking on the programme, the US ambassador to Iraq said the "overwhelming number" of troops behaved according to the law.
But Zalmay Khalilzad said abuses did exist.
"They are human beings, they violate the law, they make mistakes and they have to be held accountable and the good thing about our system is that we do hold people accountable," he said.
UK MP Bob Marshall-Andrews told the Press Association that the report confirmed "in statistical terms the appalling evidence already available in footage".
"If it is indeed systemic, then the responsibility for it must go right to the top, and that would apply to both British and American governments," he said.
A spokesman for Amnesty International UK called for a probe into the deaths in custody.
"Deaths in custody during the war on terror are a real matter of concern to us and we want to see the US and its allies allowing a full independent and impartial investigation into these deaths, as well as mounting incidents of alleged torture and other mistreatment," he said.
He said Amnesty had raised the issue of "overly lenient sentences" for those found guilty of mistreating prisoners.
Last week, an Australian TV channel broadcast previously unpublished images showing apparent US abuse of prisoners in Iraq's Abu Ghraib jail in 2003.