A person in Britain has been diagnosed with a lethal strain of E.coli, believed to originate in organic cucumbers.
The bacteria has killed nine people in Germany, with almost 300 people being admitted to hospital. Cases have also been reported in Sweden, Denmark and the Netherlands.
The outbreak is believed to have originated in organic cucumbers grown in Spain, although there are suggestions that the bacteria has been found in cucumbers grown in the Netherlands.
The advice now to people travelling to Germany is not to eat cucumbers, raw tomatoes or lettuce.
The British Health Protection Authority has confirmed that three German nationals currently in Britain have fallen ill. One of those cases has been confirmed as having the infection which is causing this outbreak.
A spokeswoman for the HPA said the outbreak in Germany was "very, very serious" and although the bug was infectious, there had been no reports of secondary infection yet in the UK.
Dr Dilys Morgan, head of the gastrointestinal, emerging and zoonotic infections department at the HPA, said: "The HPA is actively monitoring the situation very carefully and liaising with the authorities in Germany, the European Centre for Disease Control and the World Health Organisation as to the cause of the outbreak. E.coli bacteria like these are responsible for the outbreak across Europe.
"We are keeping a close watch for potential cases reported in England and are working with colleagues in the devolved administrations to recommend they do the same.
"In addition we are in the process of alerting health professionals to the situation and advising them to urgently investigate potential cases with a travel history to Germany."
In Germany concern is growing. The country's National Disease Control Centre has confirmed 60 new cases were reported in the last 24 hours.
A spokesman for the German consumer affairs minister Ilse Aigner said: "The European Union internal market has very strong safety rules and we expect all EU states to observe them."
He added that, for the moment, "one can only speculate about the causes" of the outbreak.
In Spain, a spokesman for the AESA food safety agency said investigations were also under way.
"The Andalusian authorities are investigating to find out where the contamination comes from and when it took place," he said.
"This type of bacteria can contaminate at the origin or during handling of the product."
There has been no report of contamination within Spain, AESA said.
Those worse hit by the infection contract HUS, a condition which can have severe effects.
British microbiologist Ron Cutler told Sky News: "It contains some very nasty toxins which can go straight to your kidneys and cause kidney failure, and it's very difficult to treat.
"For those who are treated, around 90% of treatments can be successful, but one in 10 of those people could have damaged kidneys in later life."
The Food Standards Agency has confirmed that the offending cucumbers have not been on sale at any outlets in the UK.