Fresh food could last for years

There would also be no need to refrigerate produce treated with the preservative, called, bisin, which is produced by harmless bacteria.

They say that foods like milk, sausages and sandwiches containing the agent could be on the shelves within three years.

Ready meals, opened wine and fresh salad dressing could also be safely consumed long after they were bought, say scientists.

Researchers at Minnesota University in the US discovered the substance from a culture of a harmless bacteria, Bifodobacterium longum, commonly found in the human gut.

It is the first naturally occurring agent identified that attacks so-called gram-negative bacteria such as E.coli, salmonella and listeria.

Dan Sullivan, an Irish microbiologist who now works at the university, told The Sunday Times: "It seems to be much better than anything which has gone before. It doesn't compromise nutrient quality — we are not adding a chemical, we are adding a natural ingredient."

He and his team have patented the substance in the US.

Bisin is related to nisin, which attacks gram-positive bacteria, and is used in the manufacture of processed cheeses and meats. As such, it is generally recognised as safe and would not have to be pharmacologically tested.

It would not be able to prevent fruit and vegetables from rotting, however, as they decompose in a different way.

Further research is now ongoing, looking at exactly how good it is at stopping bacteria from growing.

Meanwhile, a British wholesaler has begun to make sandwiches with a two-week shelf life, by replacing all the oxygen in the plastic packaging with nitrogen and carbon dioxide.

Ray Boggiana, a food technologist who helped develop the range for Booker, which supplies convenience stores, said: "The science is not new. It's all about using a protective atmosphere in the packaging."