Labour leader Ed Miliband has said the Prime Minister was "wrong" not to have spoken out in opposition of the Gaza crisis.
Mr Miliband branded David Cameron's silence on the killing of Palestinian citizens "inexplicable."
And he called for the British government to speak with a united voice to put pressure on both sides to end the violence.
However, Downing Street accused Mr Miliband of "playing politics" and insisted Mr Cameron had always been clear that both sides should observe a ceasefire.
The 26-day offensive 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.