Almost a million children in London are to be offered a polio vaccine to try to prevent the spread of the virus.
While there have been no confirmed cases, health officials are warning that there has been "some transmission" of the virus in the city after detecting poliovirus in sewage samples.
Polio causes paralysis in about 1% of cases, and this can lead to death. It usually causes fever, vomiting and muscle stiffness.
Officially eradicated in the UK in 2003, the last outbreak of polio in London was in the 1980s.
The last recorded case of polio in Ireland was in 1984.
The polio vaccine is given to all children in Ireland as part of the 6 in 1 vaccine at 2, 4 and 6 months of age.
The UK Health Security Agency (UKHSA) has identified 116 distinct polioviruses from 19 sewage samples from London boroughs including Barnet, Brent, Camden, Enfield, Hackney, Haringey, Islington and Waltham Forest.
It was first detected at Beckton sewage treatment works earlier this year.
Officials said levels of the virus found in sewage and their "genetic diversity" suggests "some virus transmission in these boroughs".
In a bid to get ahead of a potential outbreak, officials are to launch a rapid vaccination programme among children under ten, where there are lower levels of uptake of the vaccine.
Immunisation rates across London vary, but are on average below the 95% coverage rate the World Health Organization suggests is needed to keep polio under control.
Polio is spread mainly through contamination by faecal matter.
While there is no cure, vaccination - introduced in Ireland in 1957 - almost eliminated the wild, or naturally occurring, form of the disease.
The virus which has been found in London sewage occurs when children vaccinated with a particular kind of live vaccine - not used in Ireland or the UK - shed the virus in their faeces.
This harmless virus can transmit between unvaccinated children, and while doing so, can mutate back into a more dangerous version of the virus.
The UKHSA said the strains of the virus which have been detected in London are similar to the one found recently in an unvaccinated individual outside New York.
Britain is also expanding surveillance for polio to other sites outside London to see if the virus has spread further.
The risk to the wider population is assessed as low because most people are vaccinated even if rates are below the optimal levels to prevent spread.
Additional reporting Reuters