The incident occurred at an effluent treatment plant of the pharmaceutical unit in Kalol taluka in the afternoon, chief fire officer of Gandhinagar Mahesh Mod said.

''As the plant was shut today, the management had decided to clean the tank, which stores the factory's liquid waste before it is sent for treatment. Although there was hardly any liquid waste in the tank, the workers were unaware of the presence of toxic gas inside it,'' Mod said.

One of the workers fainted inside the tank, following which four others entered one after another to rescue him and eventually died because of the toxic fumes, he said.

The factory owners had not provided any safety equipment or masks to the workers, the official said.

''One after another, all the five workers went in and fainted after inhaling the toxic gas, which is usually found in tanks that have effluents. On learning about the incident, the Gandhinagar fire brigade rushed to the spot and pulled out the men, who were already dead,'' Mod added. The deceased are identified as Vinay, Shahi, Devendra Kumar, Ashish Kumar and Ranjan Kumar, all in the age group of 30 to 35, he said.