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 the keynote address to the UN General Assembly, Mr Cameron is 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, 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."