Researchers at the EU's Reference Laboratory for E.coli in Rome said scientific checks had failed to support the theory that contaminated vegetables were behind the outbreak.
"Alarmism over the consumption of vegetables is not justified... since laboratory analyses do not support the hypothesis that contaminated vegetables were the source of the infection," the laboratory said in a statement.
The latest news on the killer bug came as scientists discovered the strain combines a highly poisonous but common toxin with a rarely seen "glue" that binds it to patients' intestines.
A global team of researchers is continuing to work to establish the source of the infection, which has struck more than 1,700 people in 12 countries and killed 18.
It was originally suspected to have originated in raw tomatoes, cucumbers and lettuce, but researchers have still been unable to pinpoint the exact food responsible.
So far, seven people in the UK have been affected by the outbreak, including three Britons and four German nationals - with all seven having caught the infection in Germany.
Russian prime minister Vladimir Putin defended his country's ban on EU vegetables in the wake of the infection, saying he would not allow Russians to "get poisoned".
Scientists say it may take months to fully understand the characteristics of the bacteria - but fear this E.coli strain is the most toxic yet to hit a human population.
Tests so far have revealed it is part of a class of bacteria that has the ability to stick to intestinal walls, where it pumps out toxins, causing diarrhoea and vomiting.
The strain is part of a class of bacteria known as Shiga toxin-producing Escherichia E.coli, or STEC. Read More