Tony Blair has called for an immediate ceasefire after the UN Security Council unanimously passed a resolution aimed at ending the Middle East crisis.
The UN resolution calls for a full cessation of hostilities
However, he said there would still be difficulties until UN peacekeepers and Lebanese troops took control.
Foreign Secretary Margaret Beckett said she hoped for a "durable peace".
The resolution calls for a full cessation of hostilities between Israel and Hezbollah, then the withdrawal of Israeli forces from Lebanon.
The prime minister said: "The hostilities on both sides should cease immediately now that the resolution has finally been agreed by the whole of the international community.
"However, there will continue to be difficulties until it is clear that the combination of Lebanese forces and the UN multinational force can be effectively deployed in returning control of the south of Lebanon to the Lebanese government.
"This should start straight away."
The resolution was finally agreed at the UN in New York following days of diplomatic negotiations.
It authorises the deployment of a 15,000-strong UN peacekeeping force with increased powers which would work with 15,000 Lebanese troops in the south of the country.
Mr Blair added: "It is tragic that so many innocent lives, Lebanese and Israeli, have been lost over the past weeks.
"We must now take the steps necessary to ensure it is never repeated."
'Occupation and conflict'
Mrs Beckett broke off from a holiday in France to go to New York.
She said: "For far too long Lebanon has suffered from occupation, interference and conflict, and it is the civilian population, both in Lebanon and Israel, which has suffered most."
She said the UK believed it was right to wait for a resolution because the wording needed to take account of both Israel and Lebanon's positions to ensure a lasting solution.
Shadow foreign secretary William Hague has welcomed the UN resolution and he told BBC Radio Five Live it now had to be implemented fully.
He said: "That will require again the United Nations and the leading nations of the world in the UN Security Council to show that this force is going to be available as soon as possible - the International Force".
He added that it was necessary that the force should "have the clout, it will have the means and the determination to stop this happening again: to stop, for instance, the resupply of weaponry to Hezbollah once there is a ceasefire."
Liberal Democrat leader Sir Menzies Campbell said the resolution had been "too long delayed at terrible cost in lives, destruction and even the reputation of the United Nations itself".
He added: "What is now required is an unparalleled commitment from the international community to deal with the short-term issues of a ceasefire but also the long-term problem of a permanent solution."