British public health officials declared a national incident after routine surveillance of sewage in north and east London found evidence of community transmission of poliovirus for the first time.
The UK Health Security Agency (UKHSA) said waste from the wastewater works tested positive for vaccine-derived polio in February. Since then, more samples have been detected.
There have not been yet any reported cases of the disease or related paralysis. The risk to the general public is currently considered to be low.
But public health officials urged people to make sure they were up to date with their polio vaccinations in order to reduce the risk.
Vaccine-derived polio has the potential to spread, especially where vaccine uptake is lower, experts said.
If someone is not fully vaccinated, they could become paralyzed from polio.
The samples raised alarm bells because they were related to one another, and contained mutations that suggested polio is evolving as it spreads from person to person.
It is unclear how much of the virus has spread.
The UK has a generally good vaccination rate against polio, with 95 percent of five-year-olds vaccinated. In London, that rate is a bit lower, at 91.2 percent.
Most people who become infected with polio have no symptoms. Some can develop flu-like illnesses up to three weeks later. Between 1 in 100, and 1 in 1,000 cases, polio attacks nerves in the spine and the base of the brain. This can lead to paralysis, most commonly in the legs. Rarely the virus attacks muscles used for breathing, which is fatal.
The current worry is that polio could be circulating locally and spread more widely. Experts and researchers say fortunately no one has developed symptoms, which affects about 1 in 200 of infected, but that parents need to ensure their children are vaccinated.