Tony Blair says he will not accept failure to agree reforms to alleviate poverty at a world trade gathering without a "monumental struggle".
Geldof gave the UN summit 'four out of ten' on poverty
Mr Blair pledged to start calling bluffs made on tariffs and subsidies, ahead of a World Trade Organisation meeting in Hong Kong in December.
He spoke alongside campaigner Bob Geldof at the UN summit in New York.
Later Mr Blair joined former US president Bill Clinton to launch a programme aiming at tackling poverty.
The Clinton Global Initiative also aims at finding solutions to climate change and key world issues by involving politicians, business leaders, activists and academics.
Geldof said he was "not thrilled" with the progress made over pledges on debt, trade and development aid.
He gave the UN a mark of four out of ten for failing to make monumental pledges on debt, trade and development aid.
He said the best the summit had achieved was to reiterate what was agreed at the G8 summit in Gleneagles, Scotland, two months ago.
"I understand this is a process but the process should have been accelerated and added to at the UN," Geldof said.
Both he and the UK prime minister agreed world leaders could not afford to fail to achieve agreement at December's crucial WTO meeting.
They hope trade ministers can work out a new global trade pact which will cut subsidies and reduce tariffs, with an emphasis on freeing up agricultural markets for poorer countries.
Mr Blair warned world leaders there were just 100 days to go before the Hong Kong meeting.
"Everybody around the world at the moment is trying to call each other's bluff on this world trade. You know, 'we'll get rid of all our tariffs and subsidies and you get rid of yours'.
"We've got to start calling these bluffs and making sure that people understand if we end up with a failure in December that will echo right around the world.
"I am not prepared to have that, at least not without the most monumental struggle," he said.
The prime minister told BBC News the WTO talks would be "definitive, frankly, about whether the world's prepared to opt for free trade or not".
Mr Blair joined Mr Clinton to launch a programme aiming to tackle poverty, climate change and key world issues by involving politicians, business leaders, activists and academics in seeking solutions.
The ex-president said: "I don't claim for a second that the Clinton Global Initiative will solve all of these extraordinarily tough questions.
"But getting all of these people in one place to focus on these critical problems with a commitment to what each of us can really do to change them is an important first step."
Mr Blair highlighted the benefits of free trade, and again stressed the need for a positive outcome at the trade talks.
He said: "If we are serious about tackling global poverty we have got to have a successful outcome.
"The big test is collective international leadership and it is a test we have to pass."