Three contract labourers died of suffocation after they entered a manhole on a road here without any precaution to clear a blockage, police and officials said today.

The incident occurred last night when a labourer first entered the 10 ft deep manhole without any protective measures and on not hearing from him for a long time another also went inside.

Due to suffocation he shouted for help, upon which the tractor driver who had accompanied them also entered the manhole to rescue them.

All the three might have died due to suffocation because of toxic gases inside the manhole, officials said, adding that their bodies were recovered hours later.

The workers were engaged by Hyderabad-based Ramkay  Enterprises, a contractor employed by Bangalore Water Supply  and Sewerage Board (BWSSB) for the work, they said.

BWSSB officials said the workers were involved by the  contractors to clear the clogged line and the board was not intimated before starting the work as per procedures.

Also expressing surprise about the work being undertaken late in the night, they said contractors were supposed to use jetting machine to clean the manhole.

Minister for Bengaluru Development and State Town Planning K J George and Bengaluru Mayor G Padmavathi who visited the site have assured strict action against those responsible.

"It's an unfortunate incident, it should not have  happened. There are both Supreme Court and BWSSB orders that  no one should be allowed to enter manhole to either repair or  clean it," George told reporters.

He said "...merciless action will be taken against those responsible. Negligence is the main cause, despite laws labourers have been used for this work. Strict action will be  taken."

George also said that Rs 5 lakh compensation will be given from BWSSB, and the contractor will be penalised.

Police said they have registered a case and a probe was on.