The survey, conducted jointly by the UK Health Protection Agency and local authority body LACORS, also uncovered two products with levels above the EU's regulatory limit.
The survey results follow an assessment of the food safety of pre-packed mixed salads, those that also contain meat or seafood ingredients.
An EU regulation on microbiological criteria for foodstuffs requires that levels of Listeria monocytogenes should be below 102 cfu/g during the shelf life of ready-to-eat foods.
The UK study, which was also reviewed by the country's Food Standards Agency (FSA), is part of a EU-wide regulatory programme to reduce Listeria in the food chain, one that will add to the cost of food safety proceedures in the plant.
Although listeriosis is a relatively rare disease, its severity and the frequent involvement of manufactured foods in infection incidents has mde the pathogen a high priority for regulators. An important factor in foodborne listeriosis outbreaks is the pathogen's resistance to low temperatures. Listeria can grow to significant numbers at refrigeration temperatures whengiven sufficient time.
The UK survey found 95.2 per cent of pre-packaged mixed raw vegetable salads had no L. monocytogenes present. The remaining 4.8 per cent of samples did contain L. monocytogenes, and two of the samples were found to have levels above the EU proposed limit.
The findings on the two samples, which contained chicken, were reported to the local food authority involved. They collaborated with the FSA, the Health Protection Agency and the manufacturers to take the necessary enforcement and control actions.
L. monocytogenes was detected in 5.9 per cent, or 76 packages, of mixed raw vegetable salads containing meat. L. monocytogenes was detected in 3.8 per cent, or 54 samples, of mixed raw vegetable salads containing seafood. No samples had L. monocytogenes present at over 100 cfu/g.
A European Commission recommendation in March 2005 required member states to carry out a co-ordinated programme of sampling and testing of prepackaged mixed salads containing raw vegetables and other ingredients.
In total, 2686 samples of pre-packaged mixed raw vegetable salads containing meat or seafood were submitted by 298 local authorities and examined by 24 laboratories in the UK.
All the mixed raw vegetable salad samples with meat or seafood ingredients also contained a variety of other food ingredients, such as pasta, mayonnaise, eggs, cheese, rice, couscous, coleslaw, or croutons.
The survey found that about 93 per cent of the pre-packaged mixed salad samples were stored or displayed at or below 8°C, the legal maximum storage temperature for such products in England and Wales.
L. monocytogenes were present more often in mixed salad samples stored or displayed above 8°C, compared to those displayed at or below 8°C, although the finding was not statistically significant, the survey authors stated.
They found that 87 per cent of the samples had a use-by-date recorded on the packaging, of which 20 per cent also had display-by-date information.
Based on the information, about 81 per cent of the samples collected had remaining shelf-lives ranging from zero to three days. L. monocytogenes were present in samples that had use-by-dates ranging between zero to 12 days.
One of the two samples that contained L. monocytogenes at over 100 cfu/g had a remaining shelf-life of zero days, while the other sample had no expiry date.
A "display" date is an internal management tool used by some retailers to ensure that there is some product life left after sale to the consumer. Foods marked with a "use by" date are prohibited from being sold past the stamped date.