Aid agencies have urged the leading powers to exert the "highest diplomatic pressure" on Israel to lift its blockade of Gaza as the main international players gathered in London for Middle East talks.
In a joint appeal, the main UK agencies warned that Gaza was facing a "humanitarian crisis" unless Israel allowed unimpeded access to fuel and other essential supplies.
The meetings at Lancaster House will bring together the Quartet of the United Nations, the United States, Russia and the European Union as well as Israeli Foreign Minister Tzipi Livni, Palestinian Prime Minister Salam Fayyad, and foreign ministers of leading Arab states.
Much of the focus will be on securing more international funding for the Palestinian Authority, which is set to run out of cash in June unless it gets additional assistance.
More than seven billion US dollars (£3.5bn) has been pledged by donor nations, including the Arab states, but much of that money has yet to be forthcoming.
However diplomats also expect the issue of the nine-month blockade of Gaza, which the Israelis imposed following the takeover of the strip by the militant Islamists of Hamas, to be raised.
The aid agencies, including Oxfam, Christian Aid, Care International UK, Cafod and Medecins du Monde UK, said in a joint press release that conditions for ordinary people there were now "intolerable".
"Only a trickle of medicine, food, fuel and other goods is being allowed in. It has made people highly dependent on food aid, and brought the health system and basic services such as water and sanitation near to collapse," the statement said.
The agencies warned that the UN Relief and Works Agency had already been forced to suspend the distribution of food to 650,000 people due to lack of fuel while there were reports that 300,000 people had only four hours of drinking water every four days.
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Oxfam GB director Barbara Stocking said that it was now up to the Quartet - which leads for the international community on the Middle East - to take positive action to resolve the crisis.
