Gordon Brown condemned Israel's shelling of the United Nations HQ in Gaza today as "indefensible".
The UN building was set alight by the bombardment as its Secretary General, Ban Ki-moon, visited the area on a mission to secure a ceasefire.
The Prime Minister this afternoon assured Mr Ban, in a phone conversation following the incident, that Britain would "step up" its efforts for an immediate ceasefire.
And he said that British humanitarian aid would go into Gaza "immediately there is a ceasefire".
Speaking in 10 Downing Street shortly after talking to Mr Ban, Mr Brown described the UN chief as "properly angry" about the attack.
The Prime Minister said: "When the United Nations is doing such vital work - humanitarian work amongst women and children in Gaza - no-one can defend this attack by Israeli forces on the compound.
"The Israelis have now admitted it is a serious mistake. I have said to Ban Ki-moon this afternoon we will step up our efforts for an immediate ceasefire.
"I believe that the proposals that we and others have put to the Israelis and the Palestinians are proposals - about the crossings, about arms trafficking and about proper monitoring - that can be accepted.
"We have said that we will provide British - and other countries, France and Germany, have said that they will provide European - support for these things to happen.
"I hope these events will convince everybody that an immediate ceasefire is absolutely essential."