Most picnickers would brush away flies from food, thinking nothing of bugs briefly landing on their sandwiches.

But a new study suggests the insects carry far more dangerous bacteria than previously thought, meaning sandwiches are best avoided if they have been contaminated by flies.

Researchers at Penn State Eberly College of Science in the United States found that common houseflies carry salmonella, e-coli and even bacteria which lead to stomach ulcers and deadly sepsis.

The researchers say flies may have been overlooked by public health officials as a source of disease outbreaks.

Flies in urban areas were found to carry more bacteria than the countryside leading scientists to suggest avoiding city parks for a picnic, and eating food into more rural locations.

''People had some notion that there were pathogens that were carried by flies but had no idea of the extent to which this is true and the extent to which they are transferred,” said Dr Donald Bryant, Professor of Biotechnology at Penn State University.

"We believe that this may show a mechanism for pathogen transmission that has been overlooked by public health officials.

"It will really make you think twice about eating that potato salad that's been sitting out at your next picnic.