The opening sentence of Egypt’s new constitution describes the country as the river Nile’s gift to Egyptians. It is a grand claim, but one that helps explain Egypt’s indignation at the ongoing construction of a blockage on the Nile, thousands of miles upstream: the $4.7bn (£2.8bn) Grand Ethiopian Renaissance dam (Gerd).

Egyptians have long maintained that Ethiopia’s dam project will dangerously deplete its water stocks – about 95% of which are derived from the world’s longest river. A year ago, a former Egyptian water official boldly claimed that the Gerd might deprive Egypt of up to 10bn kilolitres, devastating roughly a million acres of farmland along the shores of the Nile.

Egypt’s National Water Research Centre said at the time, in what were highly contested claims.

Egyptian politicians have used such claims to portray the dam as a threat to national security, and have occasionally made ambiguous statements about the possibility of military action. For their part, the Ethiopian government sees the Gerd as a crucial developmental goal – a 6,000 megawatt source of surplus electricity that they could sell to foreign countries to boost their economy.

Last month, the saga took a fresh twist after the leak of a highly anticipated and hitherto suppressed report into the long-term effects of what would be Africa’s largest hydroelectric dam. Written by two water experts from each of the three main countries concerned – Ethiopia, Egypt and Sudan – as well as international advisers, the report was seen as a much needed means of arbitration between the parties concerned.

But for nearly a year the report’s contents were a mystery. After its submission last April, publication was suppressed at the request of one of the countries involved, enabling all concerned to make whatever claims they liked about its contents.

That should have changed at the end of March, when a leaked version (pdf) was finally published by the International Rivers Network (IRN), an independent group that campaigns against dams across the world. But rather than clarifying the dam’s impact once and for all, the report has become the latest pawn in a war of words between Egypt and Ethiopia.

IRN said it showed that “big questions remain” and called for a halt to the dam’s construction. But Ethiopian government spokesman Getachew Reda said the group was “absolutely biased”, and “part of the smear campaign organised by Egypt”. In the meantime, the dam’s construction continues apace.

The report is nuanced and complex, and does not try to quantify exactly the likely downstream effect of the dam on Egypt’s water supply. But its 48 pages nonetheless contain alarming findings. If the dam’s reservoirs are filled during years of average or above-average rainfall, says the report, the hydroelectric capacity of Egypt’s downstream Aswan High dam (Had) – which provides about 15% of Egypt’s power – could face a temporary 6% decrease. But if filled during years of below-average rainfall, the Gerd may “significantly impact on water supply to Egypt and cause the loss of power generation at Had for extended periods”.

Among other criticisms, the report warns that the dam’s foundations may need further structural support to protect against sliding. It also says Ethiopia has done little to assess the Gerd’s effect on local people, ecosystems and biodiversity. Based on these findings, the IRN concludes that the report “confirms Egypt’s concerns that the project’s impacts could be significant”, and calls for construction to cease pending better analysis.

Not all independent analysts share this view, however. According to Dr Ana Cascão, a researcher at the Stockholm International Water Institute whose doctoral thesis analysed hydropolitics in the Nile basin, Egypt fought for the report to be kept secret. Cascão argues the study is largely optimistic about the Gerd’s impacts – “and that’s why Egypt was not happy for it to be released”. It is critical about the dam’s social and environmental impact, she says, “but otherwise – in terms of dam safety and even in terms of water going downstream – the report is quite positive”.

This is because the Gerd may eventually help to reduce the build-up of sediment in downstream dams like the Had, increasing capacity. The Gerd will also help to keep the Nile’s flow – which presently fluctuates according to the amount of rainfall, potentially causing problems for downstream farmers even in Egypt – constant throughout the year. In terms of structural safety, Sudan – the country most endangered by any catastrophe at the Gerd – is satisfied with its construction.

Egypt’s interests may actually be aligned with Ethiopia’s, since Ethiopia will ultimately want to see as much water flow through the Gerd as possible in order to maximise hydroelectric power. It is, says Cascão, Sudan’s intentions that may instead cause the greatest long-term concern for Egypt. The Gerd would allow Sudan to siphon off more downstream water for farm irrigation, potentially allowing the republic to take more water from the Nile than allowed by an agreement signed with Egypt in 1959.

Sudan has achieved this leverage by engaging positively with the dam’s construction; Egypt’s only means of reaching a grand compromise may be through similar engagement.

But it may now be too late. According to the Ethiopian government, an army of 8,500 builders, working 24 hours a day, has already completed about 30% of the 1,800 sq km site.