Almost one in five take-away outlets, and one in eight restaurants and cafés, have poor hygiene and food control processes, according to a new report from the Food Standards Agency (FSA).
A survey of 5,000 premises throughout the UK by the agency found substandard outlets were a "significant" threat to public health.
Common causes of poor hygiene included inadequate hand-washing facilities, deficient storage controls and a lack of awareness among managers.
"This level of unhygienic practice is not acceptable," said a spokesman for the FSA. "We're working with local authorities to put systems in place to improve the regulation of failing outlets."
According to the FSA, the survey, which has just come to light, was carried out in the run-up to the advent of the General Food Hygiene Regulations that came into force at the start of this year. It was supposed to serve as a benchmark against which to measure future improvements.
Environmental health consultant Geoff Ward said that the majority of culprits were likely to be sole traders, rather than the large restaurant and take-away chains which benefit from more rigid hygiene controls and food care education.
Ward criticised the HACCP (hazard analysis, critical control points) procedures, compulsory under the new Food Hygiene Regulations, as being too complicated for many small take-away outlets. "Give them six simple rules to follow and that will solve 99% of hygiene problems," he said.
The survey was published as the FSA announced it was making available an additional £1m to help local authorities improve food safety education for small-sized catering businesses.