Speaking during a visit to Slough, Berkshire, Mr Cameron said: "What it is time for is unconditional, immediate, humanitarian ceasefire.
"What we are seeing is heartbreaking in terms of the loss of life, and the pictures that everyone has seen on their television screen are heart-rending. Everyone wants to see this stopped, so an immediate unconditional ceasefire is what is required."
The Prime Minister made clear he blames Hamas, the political movement that controls Gaza, for the outbreak of the current round of fighting, saying: "Hamas must stop attacking Israel with rocket attacks. That is how this started. It's completely unjustified and they need to stop as part of the ceasefire."
However, a former senior British diplomat called for talks with Hamas to bring it into the Middle East political process.
Sir John Holmes was UN emergency relief co-ordinator during the last Israeli invasion of Gaza in 2009, and said that, five years on, the current situation was "very much the same".
"I fear that, unless we escape this infernal logic of violence on both sides, in five years we will be in exactly the same situation again," he said. "Hamas is not going to go away and I think we have accept that, instead of just isolating Hamas, we need to talk to them."
Sir John, who is co-chairman of the International Rescue Committee UK, added: "I accept Israel is going to say 'Well, that is just rewarding terrorism' but that is the kind of language and attitude we have to get away from.
"The rest of the world has to say 'We can't go on like this'. We have to talk to Hamas ... Let's talk to them as we did talk to others and try to bring them into the political process and stop pretending they are going to go away."