My first experience of campaigning was volunteering for the Freedom from Hunger movement as a schoolboy.
My older brother ran a small newspaper to raise money for Oxfam's charity appeal – and I helped him.
We billed it as the only paper whose proceeds were devoted exclusively to helping the hungry.
As an 11-year-old I believed then what I believe now: that in a world so productive and fertile it shames us that so many still face a daily struggle to feed themselves and their families.
In the 1960s when I was first galvanised by this injustice and inspired by the example of President Kennedy, almost 40 per cent of the world's population went hungry. For 40 years we made steady progress and reduced the numbers underfed to 15 per cent of the world's population.
But today there is a new danger that rising food prices and the global recession will cause numbers to rise again. Last month – for the first time – the number going hungry passed one billion people. That's almost one in seven of our fellow human beings who are not guaranteed even basics such as rice and bread to eat each day. A hunger emergency looms and the world must act.
And it is not just the numbers going hungry that are soaring because of these twin crises.
An estimated three million more children could die in the developing world as their families' incomes plummet and they are unable to afford even the basics such as enough food or medicines.
Each child lost is a son, a daughter, a brother, a sister – who someone else loved, cherished and needed.
We need an urgent response from all the world leaders at this week's G8 under the leadership of Prime Minister Berlusconi – and we must deliver.
As a first step we must renew our promises to meet the Gleneagles target of increasing overseas development aid by $50bn by 2010, with half going to Africa.
The G8 must account for progress against the commitments countries have made.
We also need to tackle some of the areas where we have made least progress. It is outrageous that one woman dies in childbirth every minute. That is more than 500,000 each year, despite an agreement to tackle this in 2000.
The G8 should back the global consensus on maternal health, which supports health services free at the point of use and more doctors and nurses.
Britain is willing to front-load more of our aid to provide predictable finance for health systems in the developing world, and I call on other leaders to consider new action in this area.
But, as President Obama has identified, we now also need a major push on food and agriculture. In the poorest countries, seven out of 10 people rely on agriculture for their livelihoods, yet only 4 per cent of global aid is spent on agriculture. That is totally unacceptable and the balance must shift.
This week's G8 should work towards a common international system which allows developing countries to establish costed plans – like those being developed by the African Union's agriculture programme – and then identifies financing to support them.
Last year, the G8 called for a global partnership on agriculture and food security to mobilise governments, donors, scientists, the private sector and international organisations under the remit of the United Nations.
It would help to ensure access to food and support progress on other Millennium Development Goals.
Getting this in place is now ever more urgent.
But changing the architecture is not enough. We need real resources to reverse the trend in hunger levels we have seen in recent months.
President Obama has already announced a major new package in this area and alongside that I can announce that we will increase our aid spending on agriculture and food security to $1.8bn over the next three years.
This is not charity – it is investment in our shared future.
If we are successful, it is not impossible that over time Africa could feed the world.
Effective agriculture in the developing world would also mean less poverty, more global trade and – ultimately – lower prices for consumers, whether in a market in Marrakech or a supermarket in Southampton.
And we know that poverty and desperation is the father of extremism and terror.
So it is vital that we do not withdraw from our moral duty to eliminate hunger from the earth. No one should be going hungry today – we need to act now.