British human rights, religious and aid groups have urged Tony Blair to support calls for an immediate ceasefire in the current fighting in the Middle East.
The letter urges the PM to rethink his policy
In an open letter, 14 organisations stated the government had recognised the situation's "catastrophic impact" but had yet to use its full influence.
They say failure to back the United Nations risks civilian lives "rather than helping to protect them".
Amnesty International, Oxfam and Save the Children are among the signatories.
The 14 organisations also include: Basic, Cafod, Care International UK, Christian Aid, Islamic Relief Worldwide, Medact, the Muslim Council of Britain, the union Unison, War on Want, the Welfare Association and World Vision UK.
More than 380 Lebanese and 42 Israelis have died in nearly two weeks of conflict in Lebanon, which began after Hezbollah captured two Israeli soldiers in a cross-border raid on 12 July.
Along with US President George W Bush, Mr Blair has stated a ceasefire would not work unless it were adopted by both sides.
The letter suggests Mr Blair to use crisis talks in Rome on Wednesday to "add the British government's weight to calls for an immediate ceasefire".
"By failing to back the UN and call for an immediate ceasefire the UK government has reduced the impact of international calls for an immediate halt to the violence," it states.
It goes on to say the UK's present policy could result in the government finding itself in the "uncomfortable position of only calling for a ceasefire once one side in the conflict has achieved its military objectives".
It asks Mr Blair to rethink his policy as a "matter of urgency".
"To do anything else would be to fail to uphold the UK's responsibility to help protect the civilians dying in this conflict," it adds.