David Cameron has defended Israel's military operation in Gaza, arguing that it is an act of self-defence and that Hamas is to blame for the crisis.
"I've been clear throughout this crisis that Israel has the right to defend itself," he said in a statement to parliament.
"Those criticising Israel's response must ask themselves how they would expect their own government to react if hundreds of rockets were raining down on British cities today."
The prime minister, who spoke to Israeli leader Benjamin Netanyahu last night, said he had "repeated our recognition of Israel's right to take proportionate action to defend itself and our condemnation of Hamas's refusal to end their rocket attacks".
Watch David Cameron state his backing for Israel:
Cameron expressed sympathy for casualties in Gaza, referring to them as "heartbreaking", but added that the "root cause" of the conflict was Hamas's rocket attacks on Israel.
"The crisis was triggered by Hamas raining hundreds of rockets on Israeli cities, indiscriminately targeting civilians in contravention of all humanitarian law and norms," he said.
Cameron's comments about who was to blame for the fighting in Gaza were met with angry shouts from some MPs in the chamber.
Labour leader Ed Miliband was not present to respond to the prime minister because he was in Washington DC meeting US president Barack Obama.
Harriet Harman, Miliband's deputy, stood in for the opposition. She said Israel's "escalation" of the conflict was not in the country's long-term interests.
"We stand up for Israel's right to defend itself, but this escalation will not bring Israel long-lasting security," she argued.
"Every death of a Palestinian child will fuel the hatred, embolden Israel's enemies, and recruit more supporters to terrorist groups like Hamas."
Harman also described yesterday's events in Gaza as "bloodstained", and argued that the scale of killing in Gaza needed to be acknowledged.
"It is intolerable to see the harrowing images of hospitals overwhelmed, mortuaries overflowing, and parents devastated as they cradle their dying children. Yesterday the world stood witness to the most bloodstained day."
Sunday was the deadliest day of the conflict so far, with more than 100 Palestinians and 13 Israeli soldiers losing their lives.