The EU has confirmed it has stopped paying for fuel supplies to the only power plant in Gaza, leaving thousands of Palestinians without electricity.
The European Commission said it had withheld funding because of concerns over plans by Hamas, which controls Gaza, to tax electricity bills.
The fuel aid programme will resume once Hamas agrees not to introduce the tax.
On Sunday, the Israeli military opened a border crossing to allow fuel supplies in, but none was delivered.
The EU's decision has forced the Gaza Generating Company, which provides power for at least 25% of the coastal strip's 1.5m population, to shut down operations at the Nusseirat power plant.
In the past, the EU has paid for the Gaza Generating Company's fuel supplies as part of its humanitarian assistance to the Palestinian people.
But on Monday, the European Commission said it had stopped the fuel payments of around $9m (£4.5m) a month, after officials learned of plans by Hamas to profit from the subsidy by imposing a tax on electricity bills.
The commission also said it was concerned by the security situation on the Israel-Gaza border, especially at the Nahal Oz fuel crossing.
"We are ready to resume our support to the Gaza power plant within hours once we receive the appropriate assurances that all the funds will be exclusively used for the benefit of the Gaza population," it said in a statement.
Hamas has denied it plans to impose any such tax.
In the meantime, most Gazans have been left to cope with the hot summer without electricity.
Mosim Abu Rabadan, who runs a grocery store, said he was too poor to afford a generator.
"Without electricity the business is stopped. It's very bad business now," he said.
Hamas and the Western-backed Palestinian Authority in the West Bank have been trading accusations over who is to blame for the EU's actions.
Israel began cutting fuel supplies to Gaza after the Hamas takeover
The Islamist movement has accused President Mahmoud Abbas, the leader of the rival Fatah movement, of engineering the crisis for political gain.
Fatah officials have said Hamas is already collecting electricity revenues and has never paid for electricity supplied by Israel.
Hamas has been in charge of the Gaza Strip since June, when it seized control violently from Fatah.
It is regarded as a terrorist organisation by the US, EU and Israel, which have refused to deal with it directly until it recognises Israel and renounces violence.
The majority of Gaza's power comes via power lines from Israel, with a small proportion coming from Egypt. Israel began cutting off fuel supplies to Gaza after Hamas' takeover.