Uganda has dismissed claims that it has been secretly draining Lake Victoria to maintain electricity supplies.
Lake Victoria is an economic lifeline for many East Africans
A report by the International Rivers Network, a US-based environment group, said Uganda was taking more water than agreed from the lake to generate power.
This accounted for half of the drop in the lake's levels, which are at their lowest in 80 years, the report said.
But Ugandan officials who spoke to the BBC said a prolonged drought was the sole cause of the low water levels.
Tanzania has also blamed drought for recent power cuts because of lower hydro-electric output.
Analysts have warned of conflict, as East African nations compete for water.
'Pulling the plug'
Daniel 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.