Three people are being treated for exposure to rabies after being bitten by an infected puppy which was in quarantine.
The puppy was one of five brought by a rescue charity from Sri Lanka to the quarantine kennels in the Chingford area of north-east London.
It died on Friday in the kennels. The other four puppies have since been put down by vets.
The people bitten by the dog have now been vaccinated by doctors.
Because the dog died in quarantine, any public health risk was contained, the Health Protection Agency said.
The five puppies had been held at the centre since 17 April. Experts are now carrying out an analysis to establish whether the other puppies were suffering from rabies.
Other animals that may have come into contact with the puppy were being checked, according to the Department for Environment, Food and Rural Affairs.
But it stressed that further infections were "highly unlikely" and the UK remained "free of rabies" because the case had occurred in quarantine.
'System working'
Acting Chief Veterinary Officer Alick Simmons said: "While initial tests show that this puppy has tested positive for rabies, this shows that the system is working and the case has been picked up while the animal is in quarantine.
"We are now tracing animals that have moved from the kennels to ensure that all animals that have come into contact with the puppy are monitored."
The location of the quarantine premises has been given by officials only as the South East, but Tory MP Iain Duncan Smith said he had been told by environment minister Lord Rooker it was in his Chingford and Woodford Green constituency in Essex.
"Naturally, I am very concerned about the welfare of the staff who have been affected," he said.
"I was assured by the minister that the situation is under control."
Shadow Environment Secretary Peter Ainsworth said it was "concerning" that rabies had returned to the UK, but it appeared the quarantine system had worked.
He called for "swift action" to ensure the disease did not spread to the wider environment.
Effective treatment
Rabies is a viral disease which affects the central nervous system. Once symptoms appear it is almost always fatal, but patients can be treated with antibodies and a vaccination to fight the virus after being bitten.
The virus often takes a long, long time to do anything
Professor Hugh Pennington Bacteriologist
Professor Hugh Pennington, an expert in bacteriology at Aberdeen University, told the BBC the treatment for rabies had a high success rate.
"Basically it starts with washing the wound out with soap and water, which is highly effective at killing the virus," he said.
"Then the wound itself is infiltrated with anti-serum and finally there's the immunisation set-up where people are given injections over several days to build up their immunity.
"The virus often takes a long, long time to do anything and all these processes stop it dead in its tracks."
Rabies was eliminated from the animal population in the UK in the early 20th Century, but it continues to infect a variety of animals in other parts of the world.
Twenty cases of rabies have been reported in England and Wales since 1946, which were all imported.
A licensed bat handler died in Scotland from a rare form of rabies caught from a bat in 2002.
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