Gordon Brown is "appalled" by violence in Gaza and has urged an immediate ceasefire as Israeli air strikes continue, a spokesman said.
In a statement issued by Downing Street Mr Brown also said there could be "no military solution to this situation".
Israeli officials have vowed to continue attacks on Hamas as the UN calls for an immediate ceasefire.
Hamas says 312 Palestinians have died. The UN says 56 civilians are dead. Two Israelis have been killed by rockets.
Gaza's interior ministry and more sites linked to the militant group Hamas have been hit in a third day of strikes.
Loss of life
The UN says about 100 rockets or mortars have been fired into Israel following the attacks which began on Saturday.
They came less than a week after the expiry of a six-month-long ceasefire deal with Hamas - the militant movement which controls Gaza.
A statement issued by Downing Street said: "We are appalled by the continuing violence in Gaza and reiterate our call to Israel and Hamas for an immediate ceasefire to prevent further loss of innocent life."
It said Gordon Brown had held talks earlier with Israeli PM Ehud Olmert and Palestinian Authority President Mahmoud Abbas - whose Fatah party controls the West Bank but not the Gaza Strip.
Mr Brown had pressed for "full, unimpeded and urgent access for medical teams: A humanitarian breathing space," it said.
"There is no military solution to this situation," it added.
"We must redouble the international effort to ensure that both Israel and Palestine have the land, rights and security to live in peace."
Earlier Foreign Secretary David Miliband warned of a "very dark moment" in the Middle East peace process as Israel continued air strikes on Gaza.
He told BBC Radio 4's Today programme that he was concerned about the impact on the chances for achieving a peace agreement and the danger the raids would radicalise more people.
"This is very dangerous and a very dark moment," he said.
Foreign Secretary David Miliband calls for an "immediate ceasefire"
"We are now paying a terrible price for the slow and faltering pace of negotiations not just over the last year, probably not just over the last 15 years - and it is the fundamental need for a comprehensive settlement that is the only way to resolve this in the interests of the Palestinians or the Israelis."
The authority of Palestinian Authority president Mahmoud Abbas had to be reinforced, he added.
For the Conservatives, William Hague warned there was little leverage Britain could exert over the immediate situation in Gaza.
He told BBC Radio 4's World at One: "It is quite right for the UN Security Council to call for a ceasefire and an end to hostilities and we should all support that, but that does of course require both sides to cease hostilities.
"The evident reason for Israel's onslaught on Gaza in recent days has been the very large number of rocket attacks launched by Hamas into Israeli territory. That's a difficult thing to resolve."
But Ed Davey, for the Liberal Democrats, said: "The Israeli reaction is utterly disproportionate.
"From the standpoint of ordinary people in Gaza this is a full-scale attack, which is leaving women and children dead and thousands of innocent people suffering.
"The rocket attacks by Hamas are totally unacceptable, but Israel ought to have learnt from its attack on Lebanon which only served to strengthen the cause of extremism."
Israel says the aim of the strikes is to stop rockets and missiles being launched on the southern part of Israel.
Palestinian chief negotiator Saeb Erekat called for an immediate end to Israeli attacks, telling the BBC: "At the end of the day you don't solve such problems with military means and such large-scale attacks."
But Israel's deputy ambassador in London, Talya Lador-Fresher said security institutions and Hamas had been targeted and most casualties were "people in uniform, Hamas operatives that were busy trying to launch rockets and doing terror attacks against Israeli citizens".
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