People are being urged to ensure their polio vaccines are up to date after an outbreak of the virus was detected in UK sewage samples.
Polio, which was officially eradicated in the UK in 2003, can cause paralysis in rare cases and can be life-threatening.
Dr Vanessa Saliba, a consultant epidemiologist at the UKHSA, said: "Vaccine-derived poliovirus is rare and the risk to the public overall is extremely low.
"Vaccine-derived poliovirus has the potential to spread, particularly in communities where vaccine uptake is lower. On rare occasions it can cause paralysis in people who are not fully vaccinated so if you or your child are not up to date with your polio vaccinations it's important you contact your GP to catch up or, if unsure, check your red book.
"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.
"We are urgently investigating to better understand the extent of this transmission and the NHS has been asked to swiftly report any suspected cases to the UKHSA, though no cases have been reported or confirmed so far."
Symptoms of polio
Most people who get polio do not have symptoms but some suffer mild, flu-like issues such as a high temperature, extreme tiredness, headaches, vomiting, a stiff neck and muscle pain.
If the breathing muscles are affected, polio can be life-threatening.
Medics have now been alerted by UKHSA to look out for signs of polio paralysis.
Urgent medical attention should be sought if people experience rapid onset of weakness in a limb, which will be floppy, or difficulties with breathing.
What is polio disease?
Polio, which was officially eradicated in the UK in 2003, can cause paralysis in rare cases and can be life-threatening.
While it is normal for the virus to be picked up as isolated cases and not detected again, experts have raised the alarm after several genetically-linked viruses were found in samples between February and May.
Previously, the virus has been picked up when a person vaccinated overseas with the live oral polio vaccine (OPV) returned or travelled to the UK and briefly shed traces of the vaccine-like poliovirus in their faeces.
In one in 100 to one in 1,000 infections, the polio virus attacks the nerves in the spine and base of the brain.
This can cause paralysis, usually in the legs, that develops over hours or days.
