The Tredegar Arms on Caerphilly Road in Newport, south Wales, was inspected by food safety officers from Newport City Council on 17 July.
They found major improvement was needed in the hygienic handling of food, including preparation, cooking, reheating, cooling and storage.
Work was also found to be necessary in the management of food safety that involved system or checks in place to ensure food sold or served is safe to eat.
It also involves evidence that staff know about food safety, and the food safety officer has confidence that standards will be maintained in future.
Inspectors also noted there was improvement necessary in the cleanliness and condition of facilities and the building (including having appropriate layout, ventilation, hand-washing facilities and pest control) to enable good food hygiene.
A spokesman for Greene King said: “We pride ourselves on our normally high standards and we apologise to our customers for this unacceptable lapse.
“As soon as we received the report from the council, we took immediate action to remedy the situation.
“We have invested heavily in our kitchen and now have a new manager with a wealth of restaurant experience who started with us this week."
Seeking a fresh inspection
It added: “We are confident in the pub’s standards and are seeking a re-inspection as we hope to regain our previous five-star rating as soon as possible.”
The spokesman reassured customers that Greene King has taken the rating very seriously, acted immediately and said guests could enjoy a visit to the pub in confidence.
Meanwhile, a popular Harvester pub was given a zero rating for food hygiene for its cleanliness, condition of its restaurant and the management of food safety last month (August).
Since the rating, Harvester has been working with Hull City Council to address issues at the Oyster Catch pub at the Kingswood Retail Park in Hull, East Yorkshire.