David Cameron has arrived in New York for a two-day visit which will take in a speech to the United Nations and a high-profile appearance on US TV's Late Show With David Letterman.
Ahead of Wednesday's keynote address to the UN General Assembly, Mr Cameron was co-chairing a high-level development panel at which he will reaffirm his government's commitment to spend 0.7% of national income on aid by 2013.
Despite arguments from within his own party that the aid pledge is no longer affordable in tough economic times, Mr Cameron will insist that rich nations must not break their promises to the poorest people in the world.
He will challenge other world leaders to live up to their own commitments to support the Millennium Development Goals (MDGs), which include eliminating absolute poverty, providing universal primary education and fighting Aids, malaria and other diseases in developing countries.
Mr Cameron is co-chairing the High-Level Panel, set up by UN secretary-general Ban Ki-moon to chart a future framework for global development after the MDGs expire in 2015.
At the first meeting of the panel in New York on Tuesday evening, he will warn that fewer than half of the MDG objectives are on track for the 2015 deadline.
He will say: "We must send a clear message to everyone who signed up to Millennium Development Goals that now is the time to step up and honour those promises.
"I know there are some who say we can't afford to do that right now. They believe we have to focus on ourselves. And if that means breaking promises, then they're sorry but it just has to be done. Well, I'm sorry, but it doesn't.
"When we make a promise to the poorest people in the world we should keep it, not turn our backs on people who are trusting us to help them.
"More than 5,000 infants die every day from preventable diseases like pneumonia and diarrhoea. So to those who say we can't afford to act, I say, we can't afford to wait."
Children such as this one in Bangladesh shouldn't be dying from diarrhoea, the PM insists
While the coalition government in Britain has promised to hit the UN target of devoting 0.7% of national income to development aid by 2013, many other rich states - such as Italy, Japan, the US and Germany - remain well below this level.
Mr Cameron and his panel co-chairs - president Ellen Johnson Sirleaf of Liberia and president Susilo Yudhoyono of Indonesia - are due to deliver a blueprint for future global development to Mr Ban by the middle of next year.
At Tuesday's meeting they are expected to agree that the ambition should be to eliminate absolute poverty and that they should seek ideas from business, civil society and people who are living in poverty.
Mr Cameron is expected to tell the panel he wants its work to be "bold and ambitious" and to renew calls for aid to be more transparent, accountable and result-focused.
He will cite the UK's record of targeting its aid over the past two years to vaccinate more than 12 million children, support 5.3 million primary school students and provide emergency food assistance to 6 million.
Tuesday's events are a prelude to Wednesday's UN speech, when he will address heads of state and government from around the world at the annual opening of the global organisation's proceedings.
The gathering is expected to be dominated by unrest in Syria, tensions over Iran's nuclear programme and the fall-out from the Arab Spring uprisings in north Africa and the Middle East.
Mr Cameron is expected to use the opportunity presented by the General Assembly to meet leaders of some of the emerging Arab democracies.
Following his speech, he will go into the studio with Letterman to pre-record an interview for the influential late-night talk show, at which aides said he will "bang the drum" for tourism and investment in Britain.
He will have to be prepared for some off-the-wall questions from veteran host Letterman, who is renowned for mixing high politics with humour and livened up a pre-Olympic interview with Boris Johnson by asking the London mayor how long he had been cutting his own hair.
Oxfam chief executive Barbara Stocking said: "David Cameron's commitment to the poorest people on the planet is an example to other leaders and allows Britain to hold her head high on the world stage.
The UN's Millennium Development Goals include combating Cholera. This 14-year-old boy, suffering from the disease, was carried by relatives for four hours to hospital in Haiti
"Cutting aid is the easy option in difficult times but the morally right thing to do is find money to help those who most need it and the Prime Minister deserves huge credit for doing just that."
Brendan Cox, Save the Children's director of policy and advocacy said: "There has been great progress on the MDGs. Last week came the biggest fall ever in the number of children dying from easily preventable diseases and hunger. More youngsters than ever now have access to basic healthcare and education.
"This is concrete proof that aid is working. However, there is still a long way to go, and we now need to ensure we finish the job we started.
"Whatever the plans for the future of the MDGs, they must build on the success of what's been achieved, to ensure that the world's poorest - who have yet to benefit - are not left behind."
And WaterAid head of policy Henry Northover said: "David Cameron is right to call on the world's richest countries to keep their promises to the poor.
"Thanks to international aid and development over the last two decades, two billion more people now have access to life saving safe water. By meeting our international commitment to spend 0.7% of our national income on aid Britain can continue to lead from the front to champion getting water to 783 million who are still waiting for it."
But Ivan Lewis, Labour's shadow secretary of state for international development, said: "Naturally we welcome the prime minister's reaffirmation of the UK's commitment to spend 0.7% of gross national income on aid.
"However, when David Cameron is failing to meet his promise to enshrine this commitment in law, and when his biggest financial backer and newly appointed adviser Lord Ashcroft urged only last week that the aid budget be slashed, people will continue to worry that the Conservative Party's commitment is only skin deep.
"This makes it difficult for the prime minister to speak with authority when he urges other Governments to honour the 0.7% commitment."