Israel is failing in its duty to avoid innocent civilian casualties, UK Foreign Secretary Jack Straw has said.
More than 350 Palestinians have been injured in the operation
Mr Straw said the Israeli Government were also failing to meet their legal obligation to ensure responses to terrorism were proportionate.
He was speaking as Israel widened a two-week operation in Gaza.
The minister said he recognised the operation was in response to Qassam rocket attacks but it had killed 115 Palestinians, including many children.
The Israeli operation continued on Wednesday, with troops thrusting deeper into the town of Beit Lahiya.
The Israeli commander in Gaza said his troops had met significant resistance from homemade bombs and anti-tank rockets.
A member of Hamas militant movement was killed and two others were wounded by an Israeli missile fired at the town.
The operation is the deadliest since the start of the Palestinian uprising four years ago.
In a statement issued by the Foreign Office, Mr Straw said: "The UK unreservedly condemns all acts of terrorism including the firing of
"But Israel has an obligation under international law to ensure
that its response to terrorism is proportionate to the threat it faces, as well
as a duty to avoid innocent civilian casualties and humanitarian suffering.
is not meeting those obligations."
The foreign secretary acknowledged Israel's right to defend itself but was particularly worried by the "tragic case" of Iman al Hasan, the Palestinian schoolgirl who was shot 20 times on her way to school.
He said he was concerned aid agencies, including an arm of the United Nations, were having problems delivering humanitarian supplies to Gaza and the West Bank.
"Any blockages in these supplies will only worsen the already desperate living conditions of many of the people of Gaza," he added.
The Israeli Embassy in London has yet to respond to Mr Straw's criticisms.