"It's the bean sprouts," said Reinhard Burger, head of the Robert Koch Institute (Germany's centre for disease control).

"People who ate sprouts were nine times more likely to have bloody diarrhoea than those who did not," he added.

Burger also warned that the outbreak was not over. Even though no tests of the sprouts from a farm in Lower Saxony had come back positive, the epidemiological investigation of the pattern of the outbreak had produced enough evidence to draw the conclusion, he stated.

The institute, he added, was lifting its warning against eating cucumbers, tomatoes and lettuce, but keeping it in place for the sprouts.

Some 3,000 people have been taken ill with the German outbreak of E. coli, which involves a previously unknown strain of the bacterium.

Officials initially blamed the E. coli, which has killed 29 people, on imported cucumbers, then bean sprouts.

It has generated a crisis for EU vegetable-growers, with Spanish cucumber producers wrongly blamed for the contamination.