It is the first time in almost 40 years that transmission of the virus has been detected in the UK.
No cases of polio have been reported, but health chiefs have launched a hunt for the infected people and are targeting six unnamed boroughs in north and east London.
The UK Health Security Agency said a “national incident”-level inquiry was underway following the discovery of the virus after the routine testing of faeces from the Beckton sewage works.
It is thought that a small number of people in north or east London - probably an extended family or people known to one another - have the virus.
Parents were urged to ensure that their children had received the polio vaccine. It is given as part of a multiple vaccine injection in three doses to babies, followed by boosters for toddlers and teenagers, as part of the NHS’s routine vaccination programme.
Vaccine uptake in London is lower than the national average, at 89 per cent by age two. The target is 95 per cent.
Unvaccinated children and teenagers are most at risk from the current virus.
One senior health source said: “This is of concern but the current risk is very low.”
The “wild type” of polio can cause paralysis of the limbs or respiratory system, though this is rare - in only about one per cent of cases. The vast majority of people recover.
The current virus is a “vaccine-derived” poliovirus, meaning it derives from an oral polio vaccine given abroad but not in the UK. It is thought to be less dangerous than the “wild type” virus.
The UK polio vaccine switched from oral drops to an injectable vaccine in 2004.
It is thought the virus has been imported into the UK by a person who received the oral vaccine abroad. The virus is shed in faeces and can be spread to others in close contact.
UKHSA sources suggested that the individual was most likely to have come from Afghanistan, Pakistan or Nigeria as three countries use the oral vaccine that has been detected.
The UK poliovirus is not linked to an outbreak of the virus in Ukraine.
The virus was first discovered in routine fortnightly testing in February and again in April and more recently. It has since mutated and the cases have been found to be “genetically linked”.
The Beckton sewage works has a catchment area of almost four million Londoners. Experts say they cannot be sure how widely the virus has spread but expect only small numbers will be infected.
Dr Vanessa Saliba, consultant epidemiologist at UKHSA, said: “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 [vaccination diary].
“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.”
It is not unusual for poliovirus samples to be detected on occasion through fortnightly testing of sewage, but it is highly unusual for cases to be linked.
The last case of wild polio contracted in the UK was confirmed in 1984 and the UK was declared polio-free in 2003.
However the current transmission could threaten the UK’s polio-free status if it cannot be brought under control in the coming months.
Families with children known to be unvaccinated will be contacted by the NHS and asked to receive the jab.
Jane Clegg, chief nurse for the NHS in London, said: “The majority of Londoners are fully protected against polio and won’t need to take any further action, but the NHS will begin reaching out to parents of children aged under five in London who are not up-to-date with their polio vaccinations to invite them to get protected.”
Vaccine viruses can spread in under-vaccinated communities from person to person through poor hand hygiene - such as not washing hands after using the lavatory - and water and food contamination. They can also spread - less frequently - through coughs and sneezes.