It is thought someone vaccinated with the live polio vaccine - which uses a weakened version of the virus - abroad travelled to the UK and shed part of the pathogen in their stool.
But health officials insist the risk to the public overall is 'extremely low', with urgent investigations now underway to find anyone who has been infected.
https://www.dailymail.co.uk/health/article-10938639/Polio-spreading-UK-time-40-years-Evolved-version-virus-London-sewage.html