Almost a million children aged one to nine across London are to be offered a polio vaccine to try to prevent the spread of the virus.
Health officials warned there has been “some transmission” of the virus in the capital after detecting poliovirus in sewage samples.
Polio, which was officially eradicated in the UK in 2003, can cause paralysis in rare cases and can be life-threatening.
While there have been no confirmed cases, officials sounded the alarm over the rising number of samples found in sewage in London.
The UK Health Security Agency (UKHSA), working with the Medicines and Healthcare products Regulatory Agency (MHRA), found poliovirus in sewage samples in 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 total 116 samples have been found since February, the UKHSA said, although officials stressed this does not equate to 116 cases as they may have found samples from the same person on multiple occasions.
Health Secretary Steve Barclay has given the green light for a targeted vaccination programme for youngsters in London, which generally has lower levels of uptake of the vaccine.
Youngsters can also be responsible for “silent transmission” of polio, which means that they may have the virus but not show any symptoms.
Children in London aged one to nine who are not yet fully vaccinated will be offered a catch-up dose, while those who have already been fully vaccinated will be offered a booster.
They will be offered a jab by NHS London within the next four weeks with officials hoping to vaccinate all those invited within six weeks.
The programme will start in the areas where the virus has been detected in sewage and then be extended across all London boroughs.
Officials hope a rapid vaccination campaign will boost antibodies to a high degree which should interrupt transmission of the virus.
It is also hoped that the campaign will help prevent any potential cases of paralysis.