The U.K. Health Security Agency raised the alarm on Wednesday after it found several closely related polioviruses in sewage samples taken from a treatment facility in east London between February and May. That pattern of detection suggests that a form of the virus has been spreading locally, the agency said.

The detection of polio in the West is very rare and represents the latest setback for the global push to eradicate the disease, which mostly affects children and, in rare, severe cases, can cause paralysis. Global health organizations say that the eradication of polio is within reach, but that task has been frustrated by reappearances of the virus in countries where authorities thought it had been beaten back.

The type of polio detected in the London wastewater is known as vaccine-derived poliovirus type 2, a mutated form of the weakened live virus found in the oral polio vaccine that is still used in some countries. People who receive the oral vaccine shed small amounts of virus in their feces. In places that lack adequate sanitation, it sometimes mutates to resemble the naturally occurring virus and can infect people who aren’t fully immunized.

The local spread in the U.K. can likely be traced to someone who traveled from a country that still routinely uses the oral polio vaccine, said Paul Hunter, professor in medicine at the University of East Anglia. The U.K., like the U.S. and many other nations, now uses only a newer, inactivated polio vaccine that carries no risk of onward spread in their routine immunization programs.

Vaccine-derived polio has caused outbreaks in more than 40 countries since 2018 among populations with low polio vaccination rates, according to the Global Polio Eradication Initiative, or GPEI, a partnership of six organizations leading the eradication drive.

Efforts are under way to curb vaccine-derived polio by using a newer form of the oral vaccine thought to be less likely to mutate and spread. Although the inactivated vaccine is safer, the oral vaccine is preferred in countries that are still grappling with polio outbreaks because it is easy to administer and gives good gut immunity that protects against infection and virus shedding in feces, according to Nicholas Grassly, professor of vaccine epidemiology at Imperial College London.

U.K. health authorities said the wastewater detection pointed to poliovirus spreading between closely linked individuals in north and east London who are now shedding the virus in their feces. They said they hadn’t received any reports of suspected cases but are investigating whether any further transmission is taking place. Most people with polio develop few or no symptoms, but the virus can infect the brain or spinal cord, causing paralysis and sometimes death.

The last case of naturally occurring polio contracted in the U.K. was confirmed in 1984. The country was declared free of the virus by the World Health Organization in 2003. U.K. health authorities say they routinely detect one to three vaccine-like polioviruses in wastewater every year but until now these were one-off findings likely from someone who had recently received the live-virus vaccine overseas and subsequently traveled to the U.K.

“Vaccine-derived poliovirus is rare and the risk to the public overall is extremely low,” said Vanessa Saliba, consultant epidemiologist at the U.K. Health Security Agency. “Most of the U.K. population will be protected from vaccination in childhood, but in some communities with low vaccine coverage, individuals may remain at risk.”

Health services will contact parents of under-fives in London who aren’t up-to-date with their polio vaccinations in an effort to increase coverage, according to Jane Clegg, the chief nurse for the National Health Service in London. U.K. health authorities say vaccine coverage for childhood vaccines has decreased nationally and especially in parts of London in recent years. In the British capital, polio vaccination coverage is around 86.6%, according to the WHO.

Write to Denise Roland at Denise.Roland@wsj.com