The Great Western Hospital has been given a one star rating for its food hygiene following an inspection earlier this year.
Carillion, the company responsible for all food provision within the hospital apart from Costa and Boots, was told by the Food Standards Agency that major changes were required.
Hospital chiefs said they are disappointed with the outcome and expect changes to be introduced to prevent another low score at the next inspection.
All outlets at the hospital which serve food to the public have six-monthly inspections, – the results given in a five-star rating.
This is where the checks fell short: detailed records are meant to be kept for temperature checks of both food and equipment; Carillion was also told food was being poorly stored.
The company said it has since introduced changes and hopes to score much better at its next inspection.
A spokesman for Carillion said: “Carillion has taken the findings of this report very seriously and took immediate action to address the points raised.
"We undertook a thorough review of our procedures and processes to ensure that they are robust and properly implemented to fully resolve the issues identified.
“When the environmental health officer revisited we were pleased that they confirmed that we had addressed the issues identified during their initial inspection and look forward to the opportunity to have the rating reviewed during the six-monthly visit later this year.
“We remain committed to providing a high standard of service to patients and staff at Great Western Hospital."
Since the inspection the hospital and Carillion have been working with independent health specialists to stop the problems from occurring again.
A spokesman for the Great Western Hospitals NHS Foundation Trust said: “We expect the highest standards from Carillion in all the services they provide to our patients, staff and visitors, and we were clearly very disappointed by the findings of the environmental health team back in April.
“Carillion have assured us that they have addressed the issues raised and that more recent visits from the environmental health team and independent inspectors have confirmed improvements have been made.
“We have good quality modern facilities at the GWH, which are designed to meet the very highest standards of food hygiene and we will not accept anything less.
"We continue to work closely with Carillion and The Hospital Company (our Private Finance Initiative provider) to ensure improvements are maintained and we look forward to future inspections.”