Gordon Brown intensified the pressure on Israel last night to pull out of Gaza, warning that the Middle East faced its "darkest moment" following the strike on the United Nations school.

The Prime Minister insisted: "I am hopeful that the basis on which an immediate ceasefire can take place can be found."

He called for international action to open the crossings into Gaza and for the tunnels from Egypt that Israel says are used to smuggle weapons and ammunition to be closed.

He also urged a new agreement to guarantee "security to both the Palestinian people and the Israeli people".

Mr Brown said: "That is the basis on which I believe an immediate and sustainable ceasefire can happen."

With the crisis being discussed in the United Nations, the Prime Minister said proposals for ending the bloodshed in the Gaza strip were being circulated among world leaders. He said he had discussed the situation with Egypt and Turkey.

Deploring the deaths in the UN school, he said: "This is a humanitarian crisis. This is the darkest moment yet for the Middle East and it affects the whole world.

"It's because of that that we must get humanitarian aid in, as we are promising.

"And it's also because of the violence of war that we must work as hard as possible in the next few hours and days if necessary so there is an immediate and sustainable ceasefire."

William Hague, the shadow Foreign Secretary, said: "These events demonstrate that the need to secure a ceasefire on both sides and prevent further loss of life is more urgent than ever."

More than 100 British MPs have signed a statement demanding an end to the military action by Israel. They are calling for an immediate ceasefire, an embargo on the supply of military equipment to both sides and urgent intervention by the international community to end a growing humanitarian catastrophe which is unfolding.

Richard Burden, Chairman of the Britain-Palestine All Party Parliamentary Group, said: "The continuing slaughter in Gaza is an outrage. The international community has to make clear that respecting its calls for a ceasefire is not simply an optional extra for those launching attacks on either side."