The virus that causes polio has been detected in a concerning number of sewage samples in London, health officials have said.
The disease was common in the UK in the 1950s but was eliminated by 2003.
The UK Health Security Agency (UKHSA) says it was probably imported to London by someone who was recently vaccinated overseas with a live form of the virus.
It says the risk is low, but parents should ensure their children have been fully immunised against the disease.
Polio vaccine
"Most of the UK population will be protected from vaccination in childhood, but in some communities with low vaccine coverage, individuals may remain at risk," said Dr Vanessa Saliba, consultant epidemiologist at UKHSA.
An inactivated polio vaccine is used in the UK as part of the routine childhood programme. It is given to children three times before the age of one, and then again at three and 14 years of age.
Health authorities have now declared a national incident and informed the World Health Organization (WHO) of the situation.
Sewage detection
Over the past four months, the UKHSA has found the polio virus in samples collected from the Beckton sewage works, which serves a population of four million in north and east London.
Scientists believe the virus originated from someone who was immunised abroad with the live oral polio vaccine, which hasn't been used in the UK since 2004.
That person then shed traces of the virus from their gut which were detected by the sewage sampling.
Vaccine-derived polio
In rare cases, that form of the virus can then be transmitted to others and mutate into what is known as "vaccine-derived" polio.
Although weaker than the original or "wild" form of the disease, it can still cause serious illness, including paralysis, in people who are unvaccinated.
A tiny number of samples of the polio virus are detected each year in sewage surveillance, but this is the first time that a cluster of genetically-linked samples has been found repeatedly over a period of months.
What is polio?
Polio is a rare disease and is spread after a person does not wash their hands properly after using the toilet and then touches food or water consumed by others.
In more rare cases, it can be spread by coughing and sneezing.
Most people don't have any symptoms and will fight off the virus without even realising they had it.
But a small number will experience flu-like symptoms for up to three weeks.
Rare symptoms
In a very small number of cases, the polio virus attacks the nerves in the spine and base of the brain.
This can cause paralysis, usually in the legs. If the breathing muscles are affected, it can be life-threatening.