Uganda has been taking more water than agreed from Lake Victoria to generate power, accounting for half of the drop in the lake's levels, a report says.
Lake Victoria is an economic lifeline for many East Africans
Uganda and Tanzania have blamed drought for recent power cuts because of lower hydro-electric output.
But water engineer Daniel Kull says the drought has caused only half of the drop in Lake Victoria's water levels - which are the lowest in 80 years.
Analysts have warned of conflict, as East African nations compete for water.
'Pulling the plug'
Mr Kull, a hydrologist who has worked with the UN's International Strategy for Disaster Reduction in Nairobi, says that in the past two years, Uganda has been taking 55% more water from Lake Victoria than it is supposed to under a colonial-era agreement.
The water is being used to generate hydro-electric power in the Nalubaale dam, where Lake Victoria flows into the Victoria Nile.
"Today's lake levels would be around 45 centimetres higher," if Uganda had stuck to its agreed water use, he wrote in a report for the California-based environmental lobby group, International Rivers Network, which has been cited in the New Scientist journal.
"This dam complex is pulling the plug on Lake Victoria," says Frank Muramuzi of Uganda's National Association of Professional Environmentalists.
The use of the Nile waters is strictly governed by a treaty between the countries it runs through.
Egypt gets most of the water but other countries such as Ethiopia, Uganda and Kenya want to change the terms of this agreement.