Three people are being treated for exposure to rabies after they were bitten by a puppy infected with the virus, the Health Protection Agency (HPA) said.
They were bitten while at a quarantine centre where the dog was being held after it was brought into the country from Sri Lanka.
The HPA said any risk to public health had been contained and those bitten were receiving treatment which is "highly effective" in preventing rabies.
Dr Dilys Morgan, a rabies expert from the HPA, said: "This animal died whilst in quarantine which has effectively contained any public health risk.
"We understand that three individuals connected to the quarantine centre and rescue centre were bitten by the animal and all have received or will be receiving prompt protective treatment with appropriate vaccination.
"Even if someone has been bitten by an animal with rabies, prompt post-exposure treatment following the bite is highly effective in preventing rabies."
Checks will be made on other animals that may have come into contact with the infected puppy - including those which have since left the centre, the Department for Environment, Food and Rural Affairs (Defra) said.
But it stressed that the likelihood of further infections was low.
The dog, which died on Thursday, had been held in licensed quarantine premises in the South East since April 18.
After initial tests showed up a strain of rabies, a full investigation is now under way.
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.
"This means it is contained and the UK's rabies free status remains in place.
"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.
"However, any risk of the disease having spread to other animals is highly unlikely."
Defra said the UK remained "free of rabies" because the case had occurred in quarantine.
Officials have not confirmed the location of the incident but Tory MP Iain Duncan Smith said he had been informed it was in his Chingford and Woodford Green constituency in Essex.
He received a call yesterday evening from environment minister Lord (Jeff) Rooker.
The former Conservative leader said: "Naturally, I am very concerned about the welfare of the staff who have been affected.
"I was assured by the minister that the situation is under control."
But he described it as "disturbing and worrying".
Shadow environment secretary Peter Ainsworth said: "Clearly it is concerning that rabies has returned to the UK. However, it appears that the quarantine system has worked.
"It will of course be essential to ensure that any animals which have been in contact with this infected animal are traced as quickly as possible.
"Swift action must be taken to ensure there is no possibility of this disease spreading to the wider environment."Reuse content