Indian officials are investigating the leak of a radioactive substance into drinking water at an atomic power plant in the south of the country.

Fifty-five workers at the Kaiga plant needed medical treatment for excessive exposure to radiation after tritium contaminated a water cooler.

Officials said the leak might have been deliberate.

"Mischief is not ruled out. Investigations are on," plant director J P Gupta told Reuters news agency.

The 55 workers had returned to their duties at the plant, Mr Gupta added.

"This incident has in no way affected the public, safety, health and environment."

The highly protected Kaiga plant is on the west coast of India, 450km (280 miles) from Bangalore.

Tritium, also known as Hydrogen-3, is used in research, fusion reactors and neutron generators.