German experts Thursday identified Spanish cucumbers as the source of a virulent super-bacterium that has killed three people and left hundreds ill.

Hamburg's hygiene institute discovered the bacterium on three cucumbers from Spain, the city's Health Senator Cornelia Pruefer-Storcks said.

'It cannot be ruled out that other food produce is a possible source of infection,' Pruefer-Storcks said.

By Thursday, three women had died and more than 200 people had fallen gravely ill after contracting a new sub-strain of E. coli, enterohaemorrhagic Escherichia coli (EHEC), according to the Robert Koch Institute in Berlin, Germany's infectious diseases centre.

Symptoms of those gravely ill include internal bleeding, diarrhoea and kidney failure.

Hundreds more were infected with the bacteria, which usually only affects children, but were experiencing less severe symptoms. The current outbreak is unusual for causing potentially fatal symptoms in adults, mostly women.

Researchers at Muenster University identified the mutated EHEC strain, and said they were baffled as it had never caused an outbreak before.

'It surprised us completely that the strain has caused such serious illness in such a short period of time,' said Professor Helge Karch. One quirk was its ability to produce substances that disable antibiotics.

It could take another week for the researchers to unravel the DNA structure, which could help explain why it is affecting young women in particular. They are also developing a quick test for the disease.

Spanish authorities reacted cautiously to the findings that the bacterium had been traced to their produce, and the head of the state food security agency said they had not yet been officially informed.

No traces of EHEC had been reported in Spain, the agency added.

German authorities repeated an earlier warning against eating lettuce, cucumbers and raw tomatoes, and said the discovery of EHEC in Spanish cucumbers did not rule out its presence in other vegetables.

The environment agency was also testing if the bacterium could contaminate drinking water.

Several cases of EHEC sickness were also confirmed in Denmark, the Netherlands, Britain and Sweden. All patients had visited Germany prior to falling ill.

Swedish health authorities warned people to be careful visiting Germany, after several 'seriously ill' people had returned carrying the bug, including a group of 12 golf players who had visited northern Germany.

On average, between 800 and 1,200 people contract the EHEC bug annually. The Robert Koch institute said they were currently experiencing that number of new cases per week, adding that the strain was highly contagious. Between 10 and 100 germs were enough to fall ill.