LONDON, Aug 10 (Reuters) - Britain is launching a polio vaccine booster campaign for children in London aged below 10, after confirming that the virus is spreading in the capital for the first time since the 1980s.

The UK Health Security Agency has identified 116 polioviruses from 19 sewage samples this year in London. It first raised the alert on finding the virus in sewage samples in June. 

The levels of poliovirus found since and the genetic diversity indicated that transmission was taking place in a number of London boroughs, the agency said on Wednesday.

No cases have yet been identified but, in a bid to get ahead of a potential outbreak, GPs will now invite children aged 1-9 for booster vaccines, alongside a wider catch-up campaign already announced. Immunization 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, spread mainly through contamination by faecal matter, used to kill and paralyse thousands of children annually worldwide. There is no cure, but vaccination brought the world close to ending the wild, or naturally occurring, form of the disease. It paralyses less than 1% of children who are infected.