Ed Miliband has spoken out against David Cameron's handling of the crisis in Gaza, saying the Prime Minister was "wrong" not to have opposed Israel's incursion into the Palestinian enclave and branding his "silence" on the killing of Palestinian citizens "inexplicable".
Mr Miliband condemned Israel's military offensive as "wrong and unjustifiable", and called for the British Government to speak with a united voice to put pressure on both sides to end the violence.
But the Labour leader was accused of "playing politics" and misrepresenting Mr Cameron's position by Downing Street, which insisted the Prime Minister had always been clear that both sides in the conflict should observe a ceasefire.
The row came after Deputy Prime Minister Nick Clegg issued a plea for the Israeli government to halt its military operations and talk to Hamas, warning that the assault on Gaza appeared to be a "disproportionate" response to rocket attacks from the territory.
The 26-day-old offensive, launched in response to rockets fired by Hamas from Gaza into Israel, has now killed more than 1,650 Palestinians - mostly civilians - with more than 8,000 wounded, according to local officials.
Israel has lost 63 soldiers and three civilians, its highest death toll since the 2006 Lebanon war.
Israeli troops were forcing deeper into Gaza following the reported capture of Second Lieutenant Hadar Goldin.
Hamas has denied it is holding Lt Goldin - who is believed to have spent some years living in Cambridge - and suggested he may have been killed by an Israeli strike.
In a statement to the House of Commons last month, Mr Cameron voiced "grave concern" about the death toll in Gaza but stressed that Israel had a right to defend itself and accused Hamas of triggering the crisis.
Mr Miliband said that the Prime Minister was "right to say that Hamas is an appalling terrorist organisation" whose "wholly unjustified" rocket attacks and construction of tunnels for terrorist purposes had shown its murderous intent towards Israel.
But the Labour leader added: "The Prime Minister is wrong not to have opposed Israel's incursion into Gaza.
"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."
A Downing Street spokesman said in response: "The PM has been clear that both sides in the Gaza conflict need to observe a ceasefire.
"We are shocked that Ed Miliband would seek to misrepresent that position and play politics with such a serious issue."
Mr Miliband said the Government must "develop a collective response, not a differentiated one and... speak with one voice".
"We need the clear and unequivocal message that has not so far been provided to be sent from Britain to both sides in this conflict," said Mr Miliband.
"David Cameron and the Cabinet must put Britain in a leading role in pressuring both sides now to end the violence."
The Government should condemn violence from both sides, "not just by Hamas", join the European Union (EU) in putting pressure on both Israel and Hamas for a ceasefire and engage with the efforts of US secretary of state John Kerry and the Arab League to re-establish a peace process, he said.
Writing in The Guardian today, Mr Clegg called for direct talks between Israel and Hamas, which has ruled Gaza since 2007, but is regarded by Jerusalem - as well as the US and EU - as a terrorist organisation.
Mr Clegg said it was "difficult to deny that Israel's military action appears disproportionate and, combined with the Gaza blockade, is resulting in the collective suffering of the Palestinian people."
Hamas too "cannot escape blame" for the situation, as it has shown itself willing to sacrifice its own people by placing fighters and military equipment among the civilian population, he said.
"If Israel wants to secure lasting safety for its people, it must use political will, not military might, to break the cycle of violence," said the Liberal Democrat leader.
"It is time for the Israeli government to talk to the Hamas political leadership in Gaza.
"Israel's refusal to engage with president Mahmoud Abbas's new unity government, because it includes Hamas, must be reversed."
Britain is making a further £3 million available to allow a rapid response by aid workers in Gaza to what International Development Secretary Justine Greening described as "nothing short of a humanitarian catastrophe".
The activation of Britain's Rapid Response Facility - which brings total UK aid in the current crisis to £13 million - will allow pre-approved non-governmental organisations (NGOs) to access funds within a few days.
Priority is being given to projects to provide clean water and sanitation following extreme water shortages, as well as emergency healthcare, clearance of unexploded ordnance and counselling and care for civilians, particularly women and children.
The UK's Department for International Development said that since the Israeli offensive began on July 8, 136 schools - some serving as shelters - 24 hospitals and clinics and 25 ambulances have been damaged or destroyed, while eight UN aid workers and at least two Palestinian Red Crescent volunteers have now been reported as killed.
Some 40% of the sixth-most densely populated area on Earth is now a war zone, with a quarter of the Gazan population displaced.
Ms Greening said: "What is happening in Gaza is nothing short of a humanitarian catastrophe.
"Families are being driven from their homes by the fighting and the number seeking shelter and safety keeps growing.
"The £10 million we have released over the last two weeks is already helping to save lives and the UK will continue to stand by those in desperate need.
"Humanitarian workers putting their own lives at risk must be able to get on with their life-saving work.
"All sides to the fighting must give them safe, unimpeded access and we will keep pushing hard for a lasting and durable ceasefire."
Respect MP George Galloway has written to House of Commons Speaker John Bercow urging him to recall Parliament to discuss the situation in Gaza.
The Bradford West MP said: " We cannot stand by in the face of this murderous carnage.
"The British Parliament has to discuss this now and come out with a clear and unequivocal condemnation of the slaughter of civilians, principally women and children."