A deepening drought in the Middle East is aggravating a dispute over water resources after the World Bank found that Israel is taking four times as much water as the Palestinians from a vital shared aquifer.

The region faces a fifth consecutive year of drought this summer, but the World Bank report found huge disparities in water use between Israelis and Palestinians, although both share the mountain aquifer that runs the length of the occupied West Bank. Palestinians have access to only a fifth of the water supply, while Israel, which controls the area, takes the rest, the bank said.

Israelis use 240 cubic metres of water a person each year, against 75 cubic metres for West Bank Palestinians and 125 for Gazans, the bank said. Increasingly, West Bank Palestinians must rely on water bought from the Israeli national water company, Mekorot.

In some areas of the West Bank, Palestinians are surviving on as little as 10 to 15 litres a person each day, which is at or below humanitarian disaster response levels recommended to avoid epidemics. In Gaza, where Palestinians rely on an aquifer that has become increasingly saline and polluted, the situation is worse. Only 5%-10% of the available water is clean enough to drink.

The World Bank report, published last month, provoked sharp criticism from Israel, which disputed the figures and the scale of the problem on the Palestinian side. But others have welcomed the study and its findings.

Gidon Bromberg, the Israeli head of Friends of the Earth Middle East, said there was a clear failure to meet basic water needs for both Israelis and Palestinians, and that Israelis were taking "the lion's share".

"The bottom line is there is a severe water crisis out there, predominantly on the Palestinian side, and it will be felt even worse this coming summer," Bromberg said at a conference on the issue in Jerusalem.

He said the Joint Water Committee, established in 1995 with Israelis and Palestinians as an interim measure under the Oslo peace accords, had failed to produce results and needed reform.

The World Bank report said the hopes that the Oslo accords might bring water resources for a viable Palestinian state and improve the life of Palestinians had "only very partially been realised".

It said failings in water resource and management and chronic underinvestment were to blame. In Gaza, the continued Israeli economic blockade played a key role in preventing maintenance and construction of sewage and water projects. In the West Bank, Israeli military controls over the Palestinians were a factor, with Palestinians still waiting for approval on 143 water projects.

"We consider that the efficiency of our aid in the current situation is compromised," said Pier Mantovani, a Middle East water specialist for the World Bank, which is an important source of aid for the Palestinians.

Most went on short-term emergency projects with limited long-term strategic value. It was a "piecemeal, ad hoc" approach, he said.

Yossi Dreisen, a former official and now adviser at the Israeli water authority, disputed the Bank's findings and said many remarks in the report were "not correct". He produced figures suggesting Israeli water consumption per person had fallen since 1967, when Israel captured and occupied the West Bank, while Palestinian consumption had risen.

Israel argues that the water problem should be solved by finding new sources, through desalination and water treatment.

"There is not enough water in this area," said Dreisen. "Something must be done. The solution where one is giving water to the other is not acceptable to us."

However, Fuad Bateh, an adviser to the Palestinian water authority, said Israel continued to have obligations under international law as the occupying power and should allow Palestinians water resources through an "equitable and reasonable allocation in accordance with international law".

He accepted that there was a lack of institutional development and capacity on the Palestinian side, but he said the Palestinians were caught in an unequal, asymmetric dispute. Palestinians had not been allowed to develop any new production wells in the West Bank since the 1967 war.

"Palestinians have no say in the Israeli development of these shared, trans-boundary, water resources," he said. "It is a situation in which Israel has a de facto veto over Palestinian water development."