A street was closed and shops evacuated after a suspected gas leak in Taunton.
Several shops have been evacuated
East Street was closed by emergency services on Thursday morning after a strange smell was reported by members of the public.
It was thought that pools of flammable hydrogen sulphide had been disturbed in the drainage system.
But later it emerged that there was no risk to humans from the gas, that smells like stink bombs. But there is the risk the gas could ignite.
An Avon and Somerset Police spokesman said: "Concerned members of the public would appear to have smelled a substance being used in drains and pipes by Wessex Water and raised the alarm in good faith.
"As a result, a further build up of non-hazardous, but potentially flammable gas was discovered.
"Experts are again happy this poses no risk, but as a precaution, staff from Wessex Water are currently involved in ensuring all of this gas is allowed to escape from pipes and drains."
Earlier Dave Robinson, of the Devon & Somerset Fire & Rescue Service, told BBC Somerset Sound: "We had to close off part of the town centre because of some smells that people have come across.
Traffic was congested in the town as a result of the leak
"We think what may have happened is that work may have displaced some pools of hydrogen sulphide in the drainage system.
"Hydrogen sulphide is a natural phenomenon in drains: it smells like rotten eggs and the classic stink bomb smell school kids know.
"But unfortunately it's a flammable gas, so if it gets to a certain concentration in basements, we've got the risk of flash fires.
"So we're committing crews with specialist equipment to determine what the threshold limit is so we can make the situation safe and allow the public back in."