World Health Organisation says fatal E coli is a mutant blend of two different varieties and has never been seen before

A new and more virulent strain of the E coli bacterium caused the outbreak that has killed 17 people and left more than 1,500 ill across Europe, the World Health Organisation has announced.

Hilde Kruse, a food safety expert at the WHO, told the Associated Press it was "a unique strain that has never been isolated from patients before ... [its characteristics] make it more virulent and toxin-producing".

According to the Health Protection Agency three British nationals have been infected as well as four Germans in the UK. All are believed to have caught it in Germany. Three are believed to have developed haemolytic uraemic syndrome, a rare and severe kidney complication that destroys red blood cells and can affect the central nervous system.

The HPA has said it is working with the Food Standards Agency and there is no evidence of suspect produce being distributed in the UK.

As the number of cases continues to rise, Russia has extended its ban on imports of raw vegetables from the European Union - a move condemned by Brussels as "disproportionate" - and Spain is threatening legal action over the initial attempt by Germany to blame the outbreak on imported Spanish organic cucumbers.

Russia initially banned imports of raw vegetables from Germany and Spain but is extending the ban to all EU countries. Gennady Onishchenko, head of the Russian consumer protection agency Rospotrebnadzor, told the Interfax news agency the deaths "demonstrate that the much-praised European sanitary legislation which Russia is being urged to adopt does not work".

"How many more lives of European citizens does it take for European officials to tackle this problem?" he told the RIA Novosti news agency.

Exports of all vegetables, including raw vegetables, from the EU to Russia were valued at €594m euros last year, with France, Germany and Poland the biggest exporters.

European Commission spokesman Frederic Vincent said health commissioner John Dalli would be writing to Moscow "within hours" warning the ban was disproportionate.

Spain is seeking compensation from Germany of its farmers, claiming lost sales are costing €200m a week and could put 70,000 people out of work.

In Germany a health official admitted the precise source of the disease may never be traced. Reinhard Burger, head of the Robert Koch Institute, told the BBC: "I think the number of cases will come down, but how long it takes I'm not sure. It could be indeed weeks or months and I'm not sure if we will really find the source." The RKI reported 365 new cases on Wednesday and said a quarter involved a life-threatening complication.

At the European Centre for Disease Prevention and Control, Denis Coulombier, head of surveillance and response, said there was a strong link between the disease and consuming fresh vegetables.

"To have such a high number of severe cases means that probably there was a huge contamination at some junction," he told Reuters. "That could have been anywhere from the farm to the fork - in transport, packaging, cleaning, at wholesalers or retailers - anywhere along that food chain."

In Britain the HPA is urging travellers to Germany to avoid eating raw tomatoes, cucumbers or leafy salad, especially in the north of the country, and anyone returning with symptoms including bloody diarrhoea is being told to seek urgent medical attention.

Dr Dilys Morgan, head of the HPA 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."

Professor Hugh Pennington, a microbiologist from the University of Aberdeeen, told the BBC that the outbreak was unusual because it didn't seem to be affecting young children. "Children under five have had a very hard time with this kind of bug in the past. They seem to be escaping it - maybe just due to the nature of the food that's causing the problem."