Tony Blair has been urged to use his influence to increase support for an international deal to stop genocide.
The PM is being urged to influence President Bush on genocide
The charity Oxfam has praised the UK's commitment to the deal but hopes the PM will persuade less willing states.
India, Russia and Brazil have attempted to block the agreement and the US has tried to dilute it.
The pact, which would oblige countries to intervene when there is evidence of genocide in another nation, is to be tabled at a UN Summit next month.
Final negotiations over the agenda for the UN's meeting in New York - set to be the biggest ever summit of world leaders - will begin with Oxfam seeking to safeguard the proposals for international cooperation to respond to mass killings.
Oxfam director Barbara Stocking said: "This is an opportunity for the prime minister to show his commitment to a progressive foreign policy agenda.
"We're urging Britain to use every diplomatic resource at its disposal to secure an agreement designed to stop future genocides," she continued.
In particular Ms Stocking underlined the importance of Mr Blair's special relationship with President Bush in gaining US support for the plan.
So far the EU, Japan, Canada, New Zealand, South Africa, Rwanda and Kenya, as well as the UK have given their backing to the deal.
The current draft of the scheme states the signatory governments would, "share responsibility to take collective action in a timely and decisive manner" to protect against large-scale killing, war crimes, ethnic cleansing and crimes against humanity.
Oxfam has warned that if governments fail to commit to the collective arrangement they were effectively prepared to accept repetitions of the Rwanda massacre of 1994 when an estimated 800,000 people died.