HUNDREDS of council workers were evacuated from their offices for two hours yesterday morning, after fumes caused by a chemical reaction set off an alarm.

Durham and Darlington Fire and Rescue Brigade was called to Darlington Town Hall just before 11am.

It is thought that the reaction came when two cleaning products that were not supposed to be mixed, collected in a blocked drain in the building's basement.

Three fire engines attended the incident, as well as the special incident unit from Durham.

Council spokesman Steve Jones said: "Between 500 and 600 people were evacuated at 10.30am. This was a smooth operation, as we have regular fire drills."

Fire station officer Steve Owers confirmed the cause.

"The fire alarm was activated by fumes from a sink downstairs," he said.

"There was a strong chemical smell coming from the room, so our officers wore breathing apparatus.

"It could have been caused by a number of substances. The fluid was mopped up and the area sealed.

"No one was in immediate danger as there was no one working in that area at the time.

"Everyone had been evacuated by the time we arrived."

Employees stayed outside while fire officers ensured the fumes had dispersed.