"People could be putting themselves at risk by not washing these vegetables. There have been a lot of outbreaks linked to ready-to-eat vegetables and herbs, often those shipped in from other countries," the Scotsman quoted him as saying in the agency's magazine, Health Protection Matters.
"Ready-to-eat products and fresh herbs are a common cause of salmonellosis, and these infections appear to be on the increase," he added.
Professor Threlfall said that there had been many incidents when prepared salad had led to outbreaks of salmonella. He also highlighted 123 cases of the infection reported in 2004, which were linked to fast-food restaurants.
"Although the majority of infections in the UK are associated with raw food and are preventable by proper cooking procedures, ready-to-eat foods in various guises are responsible for an increasing number of infections," he wrote in the magazine.
Professor Threlfall further revealed that ready-to-eat salad bags had been recalled by supermarkets amid concern over possible salmonella contamination in the past year.
He insists that consumers must wash 'ready-to-eat' foods despite any instructions to the contrary.
The food industry in the UK has created a 400-million-pound market of bagged salads that are preferred by people who lack the time to wash and prepare it on their own.
Representatives of the industry have said that they disbelieve Prof Threlfall's proposition. They maintained that their products are bagged only after passing through the cleaning process, and that they do not require a re-wash.
"The wash the salad gets is as good as any wash you would give in the home, and washing it again at home is not going to make a substantive difference to the safety of the product," said David Barney of the Fresh Prepared Salads Producer Group, which represents UK salad companies.
However, Prof. Threlfall said that the cleaning processes to wash the salad used very dilute concentrations of chlorine or just water to protect the taste, due to which some bugs may still remain.
"We are trying to alert people about this," he said.
He has also revealed that the salad companies are aware of the problem, and that the HPA is discussing with them how to improve the situation.
"At the moment, the main message we are trying to put across is that ready-to-eat salad vegetables could have certain organisms on them and that you should give them a quick rinse before you eat them," Prof Threlfall said.
The Food Standards Agency (FSA) also supports the view that extra precautions may be necessary with pre-washed products.
"Our advice is to wash all lettuce, including bagged lettuce, when you get it home. We will review this advice if we receive extra evidence and reassurances from the industry about their cleaning processes," a spokesman said.