Operations at two hospitals were suspended after surgical kits contaminated with human flesh were discovered by medical staff.
BBC London has learned the first contaminated kit was found at Hillingdon Hospital on Wednesday.
Operations continued until Thursday, by which time 24 contaminated kits had been found, Hillingdon NHS Trust said.
Private company IH Sterile Services, which cleans the hospital's equipment, has apologised.
The company, which is owned by Sunlight Clinical Solutions, delivered 200 kits for general use by surgeons which were supposed to have been cleaned.
Contaminated equipment was also found at Mount Vernon Hospital, also run by Hillingdon NHS.
Operations were suspended there from Thursday to Friday.
A Hillingdon NHS spokesman insisted none of the equipment would have been used in surgery because staff would have noticed the small pieces of flesh and bone.
He added that operations were suspended on Thursday - a day after the first discovery - to be "100% sure" contaminated equipment was not used.
Routine operations began again on Friday morning.
An investigation is under way.
An IH Sterile Services spokesman said: "We are dismayed that one of our new, hi-tech decontamination units, operated by experienced personnel, has failed in this way.
"We have begun an urgent investigation to establish the cause of this clearly unacceptable situation.
"We are determined to correct matters and are placing additional safeguards at all our facilities, with additional inspection requirements."
He added: "We apologise wholeheartedly to the patients and hospitals we have inconvenienced."
A Department of Health spokesman said: "We are aware of the incident at Hillingdon Hospital.
"Patient safety is our top priority and hospitals have a responsibility to ensure surgical equipment is clean and safe.
"Failure to do this would be taken very seriously by the government and the regulators."
Checks carried out
IH Sterile Services provides sterilisation to numerous hospitals across England.
It currently operates from five sites - two in London, one in Kent, another in Essex and a fifth in Somerset.
The company says NHS trusts in Weston-super-Mare, north-west London and Kent outsource all their hospital and clinic sterilisation.
At least one hospital using the company, West Middlesex in London, is known to have carried out checks in light of the incident.
A spokesman said they were aware of what had happened and examined equipment which proved to be clean.