Commercial blood product at the centre of the biggest treatment scandal in the history of the NHS was approved for use after government officials were told convicts were among the paid donors and virus contamination “should be assumed”, corporate filings reveal.

The product, given to ­haemophiliacs to enable their blood to clot, was injected into thousands of patients in the UK in the 1970s and 1980s, ­including young children, who were infected with HIV and hepatitis C.

A single batch could contain plasma from 20,000 donors harvested in US prisons and some of the poorest neighbourhoods of America, where drug use and sexual infections were rife. One infected donation would contaminate the entire batch.

An inquiry headed by Sir Brian Langstaff will report next month on the scandal, which has killed about 3,000 people.

He has already concluded “wrongs were done at individual, collective and systemic levels”.

The drug firm Bayer provided 7,000 documents to the inquiry, which reveals its subsidiary, Cutter Laboratories, warned in licensing application documents in the 1970s that its commercial blood product Koate, may ­contain viruses. It said: “Since the presence or absence of hepatitis virus in Koate concentrate cannot be proven with absolute ­certainty the presence of such a virus should be assumed.”

The product, known as a factor VIII concentrate, was approved in August 1976 along with similar treatments that were a conduit for deadly viruses for several years.