A type of poliovirus derived from vaccines has been detected in London sewage samples, the World Health Organisation and British health officials said on Wednesday, adding that more analysis was underway.
No human cases of polio have been found in Britain, where the crippling disease was fully eradicated two decades ago.
The WHO said in a statement that "type 2 vaccine-derived poliovirus (VDPV2)" had been found in environmental samples in the British capital.
"It is important to note that the virus has been isolated from environmental samples only," it said, stressing that "no associated cases of paralysis have been detected."
But it warned, "any form of poliovirus anywhere is a threat to children everywhere."
A massive global effort has in recent decades come close to wiping out polio, a crippling and potentially fatal viral disease that mainly affects children under the age of five.
Cases have decreased by 99 percent since 1988, when polio was endemic in 125 countries and 350,000 cases were recorded worldwide.
The wild version of the virus now exists only in Afghanistan and Pakistan, but a type of vaccine that contains small amounts of weakened but live polio still causes occasional outbreaks elsewhere.
Oral polio vaccine (OPV) replicates in the gut and can be passed to others through faecal-contaminated water – meaning it won't hurt the child who has been vaccinated, but could infect their neighbours in places where hygiene and immunisation levels are low.
While weaker than wild poliovirus, this variant can cause serious illness and paralysis in people not vaccinated against the disease.
Globally, there were 959 confirmed cases of VDPV2 in 2020, according to the WHO.
Polio eradication expert Kathlene O'Reilly warned on Wednesday that the discovery in the London sewage samples suggests "there may be localised spread of poliovirus, most likely within individuals that are not up to date with polio immunisations". (AFP)