His claim follows a Health Protection Agency investigation into an outbreak of salad-linked Cryptosporidium infections that affected around 300 people in England and Scotland in May.
In the analysis of the exposure to different salad vegetables a significant statistical association was found between infection and the consumption of pre-cut spinach.
When specific retailers were included in the analysis, the strongest association with infection was found to be with consumption of ready to eat pre-cut mixed salad leaves from a major supermarket chain.
“Together these findings suggest that one or more types of salad vegetables could have been contaminated,” said the HPA.
Professor Pennington said the case also followed on from several in the USA where they are “very worried” about “washed and ready-to-eat” bagged salad.
Last year produce giant Dole issued a recall on its American Blend bagged salad in 10 states in those two regions, after the Tennessee Department of Health found listeria bacteria in one sample.
Demand for salad has boomed because of healthily eating campaigns. But salad is considered one of the products most likely to cause food-related illness – largely because greens are grown directly in the soil, and some pathogens can only be killed by heat or strong detergents, not just water.
Professor Pennington said: “It is generally safer to eat a burger than the salad that goes with it.
“Despite the recent horsemeat and other scandals, the meat can be traced and through a rigorous process that checks for its quality etc.
“That does not exist to the same rigour for salad. You can only make vegetables safe by cooking and you can`t obviously do that with salad.
“You could irradiate it – but that would be a `no, no` with the public. You just can`t be absolutely sure that the bagged salad you are buying – which has been put through a chemical wash to kill the bugs, is actually free of them.
“These bugs are very good at clinging on to salad and the risk from cryptosporidium, salmonella and listeria is very real.
“I would advise people to thoroughly wash salad even when it says it has been washed and is ready to eat.”
Bagged salad on sale in supermarkets is often sourced from the same suppliers for most leaf types, often with common production lines packing product for several retailers at the same time.
Professor Pennington also pointed out that a bean sprout farm in northern Germany was identified as the most likely source of many of the infections in the E. coli outbreak that left 22 people dead in 2011.
The farm, located in Uelzen, south of Hamburg, was the epicentre of the outbreak that has also made more than 2,000 people ill.
Professor Pennington, emeritus professor of bacteriology at the University of Aberdeen, said the biggest E.coli outbreak happened in Japan in 1996 and involved radish sprouts.
“My understanding is that this farm in Germany was an organic one and there are more risks with organic food. For example organic chicken has more bugs than non-organic because they spend longer in the field and have wild bird droppings on them etc.
“Vegetables are fine and safe if they are cooked in the traditional British way of boiling them to death. The only danger comes when you eat them raw.
“Sometimes the spouts are contaminated to start with and they can get contaminated when spouting in the steaming process.
“At the end of the day the responsibility falls on the people who produce food. But much of our vegetables are now grown in countries who do not necessarily have the same hygiene standards.
“At the end of the day there has to be trust who is in supplying you with your food. The consumer has no way of knowing how the food has been produced. The consumer is not in a position to know all that has gone on.”
Professor Pennington headed the investigation into the E.coli outbreak in Wishaw which claimed the lives of 20 elderly people in 1996.