The move was the latest Israeli step to increase pressure on Gaza, ruled by the Islamist Hamas since June and the source of repeated rocket attacks into the Jewish state.
Critics had accused Israel of imposing a blockade that amounts to "collective punishment" of the population of the territory after it cut fuel to Gaza's main power plant last month, leading to blackouts.
"We are trying to reduce the Gaza Strip's dependence on Israel in many fields . . . the High Court ruled that we are acting in a correct and appropriate manner," deputy Defense Minister Matan Vilnai said.
A spokesman for Vilnai said Israel would reduce electricity on one of 10 high-power lines by less than one megawatt by the end of today.
It would eventually reduce power by 5 percent of the 124 megawatts it supplies the Gaza Strip.
"This is not punishment . . . we have disengaged from them by pulling out our troops and settlers and now we want them to start looking after themselves, they have their own power station, they get power from Egypt, we want them to . . . stop relying on Israel," spokesman Eitan Ginsburg said.
Ginsburg said that apart from Israel's 124 megawatts, a power station in Gaza City produced 64 megawatts, and Egypt supplied 17 megawatts.
Asked to comment, US State Department spokesman Tom Casey said he was not familiar with the Israeli decision, but added: "We understand Israel's right to defend itself, but we do not think that actions should be taken that would infringe upon or worsen the humanitarian situation for the civilian population in Gaza."
The move followed renewed violence in Gaza. Israeli troops backed by tanks, helicopter gunships, and warplanes killed six Palestinian gunmen and a schoolteacher in a dawn raid yesterday, militant groups and hospital officials said. Israel has increased military action since Hamas claimed responsibility for a Palestinian suicide bombing on Monday.
© 2008 Reuters