The discovery in the London sewage samples suggests "there may be localised spread of polio virus, most likely within individuals that are not up to date with polio immunisations," Polio eradication expert says.
A type of polio virus derived from vaccines has been detected in London sewage samples, the World Health Organization and British health officials said.
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 on Wednesday 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 polio virus anywhere is a threat to children everywhere."
'Check vaccination histories'
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.
While weaker than wild poliovirus, this variant can cause serious illness and paralysis in people not vaccinated against the disease.
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".
"The most effective way to prevent further spread is to check vaccination histories, especially of young children, to check that polio vaccination is included," she said.