The Health Protection Agency (HPA) has said that three of the seven people in Britain who have been infected by the strain of E coli sweeping Europe have developed haemolytic uraemic syndrome (HUS), a rare and severe kidney complication that destroys red blood cells and can affect the brain and spinal cord.
The HPA confirmed that all seven patients had recently visited Germany, where an 81-year-old woman became the 18th victim of the previously unknown E coli strain when she died on Wednesday at Hamburg-Eppendorf university hospital. More than 2,000 others have so far been infected in 10 countries from eating contaminated vegetables.
The HPA reminded Britons travelling to Germany to avoid eating raw tomatoes, cucumbers and leafy salad including lettuce, especially in the north of the country, until further notice.
Anyone returning from Germany with symptoms including bloody diarrhoea should seek urgent medical attention and ensure they mention their recent travel history.
Dilys Morgan, head of the agency's gastrointestinal department, said: "The HPA continues to actively monitor the situation very carefully and we are working with the authorities in Germany and with our counterparts across Europe as to the cause of the outbreak. We have alerted health professionals to the situation and advised them to urgently investigate and report suspected cases with a travel history to Germany."
Many of those ill in hospital require transfusions of blood plasma. In Hamburg, where there have been most cases, the mayor Olaf Scholz was photographed giving blood as he made an appeal for others to follow suit.
Though there is enough blood in the bank, German health authorities are trying to pre-empt midsummer shortages. "That's why it's important to do something now while we still have enough reserves – so that we have enough to fall back on," said Scholz.
The EU continued to insist there was not enough evidence for any foodstuffs to be banned, but Germany's disease control authority, the Robert Koch Institute, has advised German consumers against eating raw tomatoes, cucumbers and lettuce.
"This recommendation applies especially to foods acquired in northern Germany and will be upheld while the outbreak investigations continue as long as the source of the infection has not been identified," the institute said.
Genetic sequencing of the bacteria behind the outbreak suggests that it is a previously unknown strain. Dr Hilde Kruse, a food safety expert at the WHO, said: "This is a unique strain that has never been isolated from patients before". She said that the new strain has "various characteristics that make it more virulent and toxin-producing".
Paul Wigley of the University of Liverpool's School of Veterinary Science, said that the E coli outbreak was caused by a type called O104, part of a group of bacteria that produces a chemical called verocytotoxin (Vtec).
"[These bacteria] normally infect people directly through animal faeces, or more usually through poorly cooked meat contaminated with the bacteria. Whilst most strains of E coli do not cause disease, Vtec are able to attach to the wall of the intestines very tightly and produce toxins.
"It is these toxins that cause damage to the gut, leading to bloody diarrhoea and may cause damage throughout the body including the kidneys.
"Animals, and in particular cattle, may carry Vtec in their intestines without disease, which may lead to the bacterium being shed in their faeces. It is most likely that the use of manure as a fertiliser in organic salad vegetable production has lead to contamination of cucumbers and other vegetables.
"Although E coli infection is most commonly associated with meat, there have been previous reports of the disease being contracted from raw vegetables."
Dr Anthony Hilton, a microbiologist at Aston University, said the pattern of infection was unusual because of the proportion of adults with HUS, a condition normally observed in children, and the bias towards females.
"E coli O104 is uncommon and that may actually help in the investigation to determine the origin and routes of transmission of this strain," he said. "Nevertheless it may be some time, if at all, that we are able to pinpoint this with any certainty."
As the number of cases rises, Russia has extended its ban on imports of raw vegetables from the EU, a move condemned by Brussels as disproportionate. Spain is threatening legal action over the initial attempt by Germany to blame the outbreak on imported Spanish organic cucumbers.
No fatalities or infections have yet been reported in Russia where Gennady Onishchenko, the head of the consumer protection agency Rospotrebnadzor, told the Interfax news agency that the deaths "demonstrate that the much-praised European sanitary legislation that Russia is being urged to adopt does not work".