Chris Grayling has lashed out at Ed Miliband after he criticised the prime minister's response to Israel's behaviour in Gaza and called on him to push for a withdrawal.
The justice secretary said Miliband's comments on Israel would make it harder for Britain to find a diplomatic solution in the crisis.
"I think we in Britain have to be enormously careful to try and work in a way that encourages a ceasefire, encourages an end to the action on both sides," he said.
"I don't think it helps if we make strong comments that are going to put us in less of a position to do that."
Downing Street accused the Labour leader of "playing politics" with the issue.
Grayling's comments came after an Israeli bomb killed ten and wounded 35 at another UN school – the third so far – in the town of Rafah.
The school had been used as a shelter for local families who had fled there from their homes after being instructed to do so by Israeli Defence Forces (IDF).
As in previous instances, the UN gave the IDF the coordinates of the school, but it was shelled anyway.
The attack triggered a fresh round of criticism from Labour, with shadow foreign secretary Douglas Alexander caling it "sickening".
He added: "The continued killing of civilians and children in Gaza in recent days represents both a moral failure and a strategic error."
“The continued killing of civilians and children in Gaza in recent days represents both a.
In his toughest words against Israel so far, UN secretary general Ban Ki-moon described it as a "moral outrage and a criminal act".
But the British response has been far warmer towards Israel, with Cameron consistently blaming Hamas for starting the conflict and stressing that it is responsible for the Israeli counter-attack.
Some analysts believe this is because Cameron was stung by criticism from top Tory donors and supporters after he criticised Israel's actions when it attacked Lebanon in 2006.
He did ratchet up his rhetoric today however, when he said the UN was "right to speak out in the way that it has" and that the loss of life had been "appalling".
"International law is clear that it's completely wrong and illegal to target schools," he said.
"The use of force always has to be proportionate."
Miliband's statement on Israel suggests he would take a much tougher stance on the country if he became prime minister.
He said on Saturday that Cameron had failed to speak clearly and unequivocally during the month-long operation.
"The prime minister is wrong not to have opposed Israel's incursion into Gaza," he said.
"And his silence on the killing of hundreds of innocent Palestinian civilians caused by Israel's military action will be inexplicable to people across Britain and internationally.
"I am a supporter of Israel and I believe in Israel's right to self-defence. But its military actions in the past two weeks have been wrong and unjustifiable."
He added: "The escalation of violence engulfing Gaza has led, and is leading, to suffering and destruction on an appalling scale, and is losing Israel friends in the international community day by day.
"Israel's present military action will increase the future threats to its security rather than countering them. Israelis rightly and justifiably want that security, yet their government's present actions instead risk simply growing a new generation bent on revenge."
There is pressure for a more critical approach to Israel's conduct from inside the Tory party as well.
Margot James, No.10 policy board adviser and aide to William Hague wrote to new foreign secretary Phillip Hammond calling on him to toughen his stance towards Israel.
"I ask that the government rethinks policy towards the conflict in Israel and the Palestinian territories," she wrote.
"The scale of suffering in Gaza is far too great, the loss of life, and particularly the lives of children and other vulnerable individuals, cannot be justified on the grounds of defence in proportion to the level of threat faced by Israel from Hamas."
Former Northern Ireland minister Sir Peter Bottomley wrote to chief whip Michael Gove saying most MPs in the centre of the Tory party were highly uncomfortable with the way Israel had conducted itself.
"We all know that Israel has the right to exist, we all know that the attacks on Israel should cease, we know that Israel's settlements and their treatment of Palestinians is provocative," he wrote.
"That's the foundation. The issue now is if Israel is relying on other people to be silent, they'll go on with a lack of proportionality and the devastation and the death."
Speaking later to the BBC, he said: "Anyone who looks at the pictures of what's going on presently in Gaza must know that the Israelis know what they're doing and what they're doing is wrong. Many Israelis know it's wrong."
Foreign secretary Hammond has admitted he had been inundated with letters from members of the public horrified at the images of dead children coming in from Gaza.
A YouGov survey from before the worst of the school bombings showed 62% of the British public believe Israel has committed war crimes in the Gaza Strip.
Protests against Israel have been commonplace, with hundreds participating in one outside the Israeli embassy last Friday. A major demonstration is planned for London this Saturday.
Over 1,750 Palestinians – overwhelmingly civilians – have been killed since the Israeli operation began, including hundreds of children. Sixty-four Israeli soldiers have also been killed.
Israel has withdrawn most of its troops from Gaza ahead of a short ceasefire, although it will keep shelling Rafah, where the UN school was situated.