The water in Bellandur lake falls under E-Class of the Central Pollution Control Board since 1990.
Officials in the Karnataka State Pollution Control Board (KSPCB) said E-Class is the lowest category and water bodies classified under it are only fit for industrial cooling and irrigation.
But officials and experts warn that the lake water should not be used for irrigation.
KSPCB chairman Lakshman told DH that they have informed the local authorities about the poor quality of water several times, but nothing has been done about it.
Samples are collected and analysed every month and they show a deterioration in quality.
The chairman spoke to DH on the sidelines of the inauguration of the new office of Environmental Health and Safety Consultants Private Limited in Bengaluru on Wednesday.
The company is recognised by the Ministry of Environment and Forests (MoEF) and is working with the National Green Tribunal in analysing air and water samples.
Mohammed F Rahman, a former consultant at National Fisheries Development Board, said that water was clean in the early 1960s and people could drink it.
Bellandur lake was Asia's highest fish producing water body. Now the main problem is the level of dissolved oxygen in the lake.
Another KSPCB official said that monthly reports are being shared with the Bangalore Development Authority, Bruhat Bengaluru Mahanagara Palike, Bangalore Water Supply and Sewerage Board and the deputy commissioner of Bengaluru Urban.
But over the years, they have not taken any action. "It is astonishing that for the last 10 years, the lake water has been in Class-E. This should have caught their attention," KSPCB officials said.