A number of hospitals and care homes have been found to serve out-of-date food, in unhygienic premises, according to an investigation.
Vulnerable patients are being put at "high" risk of food poisoning as a result of meals being prepared in mouldy kitchens, while other sites have unclean worktops, food trolleys and sinks.
Food hygiene reports obtained by the Press Association with a Freedom of Information request and data from the Food Standards Agency (FSA) also revealed poor rankings for establishments including nurseries.
Some 400 hospitals, hospices, care homes, nurseries and school clubs are currently listed as needing "major", "urgent" or "necessary" improvement.
One care home was infested with cockroaches while another had evidence of rats, while hospitals were found to be serving out-of-date food from kitchens full of flies.
The Food Hygiene Rating Scheme - which rates organisations and businesses from zero to five - is run by the FSA and councils in England, Wales and Northern Ireland.
Overall, eight health and care premises were found to have a zero rating, denoting that urgent improvement was required.
The investigation also found 187 premises have a rating of one - which means major improvement is necessary.
A further 205 health and care organisations, including six hospitals and around 100 care homes were given a score of two.
Katherine Murphy, head of the Patients Association, said the findings were "shameful" and "immensely worrying".
The Hospital Caterers Association (HCA) and the National Association of Care Catering (NACC) have both responded to the findings of the investigation.
Phil Shelley, HCA national chair, said: "The HCA welcomes the Food Standards Agency report findings that 99% of NHS hospitals nationwide are achieving a food hygiene rating of 3 or more. We are of course saddened to see some premises scored below acceptable measures and urge those involved to address this expediently if they haven't already done so.
"The HCA encourages independent assessment such as this FSA report to ensure that the highest possible standards can be met. We encourage our members to work together with inspectors to ensure that swift action is taken if required."
He added: "We are reaching out to any of our members mentioned in the report to offer them additional support to help them carry on the good work of hospitals caterers nationwide."
Neel Radia, national chair of the NACC, said: "Good food hygiene practice is an important part of the catering provision within care homes and essential to ensure the wellbeing of residents. Support and guidance is readily available from associations, including the FSA and NACC, to help catering providers ensure that their food hygiene continually meets the required standards.
"To put things in perspective, this report relates to a very small minority of the 20,000 care homes across the UK. Daily, we see many fantastic examples of best practice in care catering, from nutrition and hydration through to food hygiene, but, sadly, these never make the headlines."
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