Russia banned imports of raw vegetables from the European Union on Thursday as a deadly E.coli outbreak centred in Germany spread to more countries.

German health officials said the infections, which have killed 17 people and made more than 1,500 others ill in eight European countries, could go on for months and their precise source may never be pinned down.

Seven people in Britain have been infected with the E. coli bacteria, with all cases linked to Germany, the Health Protection Agency said. Three of those infected were British nationals who had recently travelled to Germany and four were German nationals, it said.

Meanwhile, the World Health Organization said the E. coli bacteria responsible for the outbreak is a new strain that has never been seen before. Preliminary genetic sequencing suggests the strain is a mutant form of two different E. coli bacteria, with lethal genes that could explain why the Europe-wide outbreak appears to be so massive and dangerous, the agency said.

Russia had already banned imports of vegetables from Germany and Spain over the outbreak, which German officials originally blamed on contaminated cucumbers imported from Spain before backtracking and apologising to Madrid.
   
Gennady Onishchenko, head of the Russian consumer protection agency Rospotrebnadzor, said the deaths caused by the outbreak 'demonstrate that the much-praised European sanitary legislation, which Russia is being urged to adopt, does not work,' Interfax news agency reported.

The new ban would take effect on Thursday morning, he said prompting an immediate protest from the European Union, which exported 594 million euros ($853 million) worth of vegetables to Russia in 2010, according to EU data. It was not clear what proportion of that total was raw.

Spain is threatening legal action over the crisis. It wants compensation for its farmers, who say lost sales are costing them 200 million euros ($287 million) a week and could put 70,000 people out of work.

The European Commission has said it is looking at what it could do about the impact on producers.

Reinhard Burger, head of German disease control agency the Robert Koch Institute (RKI), admitted on Thursday that the precise source of the disease outbreak may never be found.   

'I think the numbers (of cases) will come down but how long it takes, I'm not sure,' he told BBC radio. 'It could be indeed weeks or months and I'm not sure if we will really find the source.'    

The RKI reported 365 new E.coli cases on Wednesday and said a quarter of them involved a life-threatening complication of a type of E.coli known as Shiga toxin-producing E.coli (STEC).        

EU health experts say that have been shocked by the size and severity of the outbreak, which is on a scale never seen before in the region. Denis Coulombier, head of surveillance and response for the European Centre for Disease Prevention and Control (ECDC), which monitors disease in the EU, said studies so far show a strong link between disease symptoms and the consumption of 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 in an interview. 'That could have been anywhere from the farm to the fork -- in transport, packaging, cleaning, at wholesalers, or retailers -- anywhere along that food chain.' 

Gennady Onishchenko, the head of Russia's consumer protection agency Rospotrebnadzor, said it was imposing a ban 'the situation has not been brought under control in a month'.

'The sources of the infection and factors in its spread have not been established,' state-run RIA quoted him as saying. 

European Commission spokesman Frederic Vincent said the EU Health Commission John Dalli would be writing to Moscow 'within hours' to tell them the ban is disproportionate. 

The ECDC says cases of E.coli linked with this outbreak have also been reported in Sweden, where one of the 16 deaths occurred, and in Denmark, Britain, Spain and the Netherlands.  Most infections reported outside Germany involved German nationals or people who had recently travelled to the country.  - Reuters