The Healthcare Commission served Bromley Hospitals NHS Trust with an improvement notice over poor practice in tackling infection control.
Poor decontamination procedures for surgical equipment were also identified during two inspections in January.
The trust, which runs three hospitals in south London, said the commission's report will help improve its standards.
The Princess Royal University Hospital, Orpington Hospital and Beckenham Hospital are run by the trust.
"This is not acceptable to patients"
The commission carried out an unannounced inspection at the Princess Royal University Hospital and found the trust was "failing in its duty to provide and maintain a clean and appropriate environment for healthcare".
The inspectors talked to some patients who led investigators to what appeared to be bloodstains on the cot side-rail of a bed and blood splashes on a wall behind another bed.
The commission's chief executive Anna Walker said: "This is not acceptable to patients, who want to know that everything possible is being done to protect their safety and ensure a good standard of care."
'Not complacent'
Ian Wilson, chief executive of Bromley Hospitals Trust, said: "I would like to reassure the people of Bromley and surrounding areas that we provide safe clinical care at the Princess Royal University Hospital and that we take the prevention and control of infection very seriously.
"However, we are not complacent. We are keen to learn how we can improve still further and continuously strive to keep infection rates to an absolute minimum.
"The Healthcare Commission's inspection report is a valuable expert view that will help us with that."
But Geoff Martin, of campaign group Health Emergency, said: "Bromley Hospitals has one of the biggest debts anywhere in the country, of almost £100m.
"They are cutting corners on cleaning because they have been effectively bankrupted by the cost of a private finance initiative (PFI) deal at the Princess Royal.
"This is the harsh reality of PFI coming home to roost."
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