A new report has alleged that Britain's largest food chains are not meeting basic legal hygiene standards at their restaurants.
Food safety ratings given by local councils suggest that about one-third of the Japanese seafood chain Yo! Sushi's restaurants do not meet all the legal standards required of them.
Similar problems were found with 25 per cent of Pizza Hut outlets also.
In a survey of Pizza Express's 132 restaurants, it was found that 18 restaurant did not meet the standards.
The law requires all restaurants to be inspected every two years by environmental health officers, and handing out of star ratings to them on the bases of the results.
Restaurants with no stars are "very poor" with a general failure to comply with legal requirements.
One-star establishments have poor compliance while those with two stars need to make more effort to hit all the legal requirements - designed to stop the spread of bacteria that can cause gastro-intestinal diseases.
An analysis of the star rating of 1,270 outlets run by 10 of Britain's best-known restaurant chains-which included McDonald's, Burger King, KFC, Pizza Express, Gourmet Burger Kitchen, and Wagamama-shows that each of them had at least one branch that did not comply with food safety legislation.
Yo! Sushi was found to be the worst performer of the 10 chains, with eight of its 23 restaurants in the sample receiving two stars or less.
Almost one in five Pizza Hut outlets failed to score three stars, deemed as meeting all legal requirements. On the other hand, for Pizza Express, one in eight was found to have unsatisfactory problems.
An inspector who inspected Yo! Sushi in Soho, central London, in October 2006, found dirty staff changing areas, failures in defrosting and cooking, dirty floors, mice droppings on a food shelf, slime on cutting boards and no records of staff training.
"Staff understood the basics but were not attending to cleanliness," the Independent quoted Westminster Council's inspector as saying.
Professor Hugh Pennington, a leading food expert, called the findings disturbing.
"Any restaurant scoring two stars (more effort required to meet all legal requirements) is unsafe. Lower scores indicate imminent danger. I would expect those with a score of zero to have been closed on the spot," he said.
Steve Vaughan, a senior consultant at All Food Hygiene which trains food manufacturers around the world in hygiene, also expressed surprise at the survey results.
"If I had a restaurant I wouldn't tolerate a low star rating. There is a risk of food poisoning. They need an action team to sort out the issues," he said.