Seven cases of the food poisoning bacteria were diagnosed in Britain, the agency said that the mutant strain was so virulent that sufferers risked spreading the infection to friends and relations through close contact.
With more than 30,000 people travelling between Britain and Germany every day, officials fear the outbreak could take hold here.
The agency said anyone who had recently travelled to Germany should be vigilant about their personal hygiene to minimise the risk of passing on the bacteria, which can attack the kidneys with potentially fatal consequences.
The outbreak is on course to be the world’s “biggest ever”, according to one of the country’s leading microbiologists. The agency added that it was shocked by its “unprecedented” scale and severity. More than 1,600 people have been infected worldwide, mainly in northern Germany. Hundreds have been left seriously ill and at least 18 have died.
It emerged last night that the food bug has struck two Americans who had recently travelled to Hamburg. Both are expected to survive but experts warned that the bacteria could be exported to the US.
Scientists also said that the new strain appeared likely to be resistant to common antibiotics.
A spokesperson for the UK's Health Protection Agency said the organisation had not sequenced the bacterium but had agreed with the WHO finding that the E coli O104 strain associated with the outbreak "which we know to have a highly unusual combination of virulent properties, could be one that has never been seen before".
There is no evidence yet that the bacteria have appeared on British vegetables.
Stephen Smith, a clinical microbiologist at Trinity College, Dublin, said the new E coli strain was a "mongrel" combining two "nasty" types of the bacterium. He said: "It is very similar to enteroaggregative E coli which has been associated with outbreaks of watery diarrhoea, in developing nations since 1970. However, this bacterium has been recognised as a cause of diarrhoea in industrialised nations and has caused outbreaks in the US, Sweden, Britain and Germany."
The toxin produced by the bacterium binds to, and damages, kidney cells and leads to haemolytic uraemic syndrome (HUS), a rare and severe complication that destroys red blood cells and can affect the central nervous system. More than 500 cases of HUS have been reported in Germany and three cases were found in the UK in people who had recently been to Germany.
An HPA spokesperson said: "Bacteria and viruses are evolving all the time. We expect to see new strains, sometimes more virulent or resistant to antibiotics than others, and plan on that basis."
