An unparalleled number of severe food shortages has added 43 million to the number of people going hungry worldwide this year. And millions of children are now at risk of acute malnutrition, charities are warning. One week ahead of David Cameron's "hunger summit", they say that unless action is taken urgently, many more could fall victim.

For the first time in recent history, humanitarian organisations have had to respond to three serious food crises – in West Africa, Yemen and East Africa – in the past 12 months, according to Oxfam. Almost a billion people are now hungry – one in seven of the global population – and the number of acutely malnourished children has risen for the first time this decade.

But these issues are well known. When the hunger crisis hit the headlines last year, it was only after famine had already been declared in Somalia, killing an estimated 100,000 people and affecting 12 million. Needless deaths occurred and millions of extra dollars were spent simply because the international community had failed to act on early warnings.

The Harry Potter actress Bonnie Wright, 21, has just returned from Senegal, a country in the Sahel region of West and Central Africa, where more than 18 million people are threatened by food shortages. She told The Independent on Sunday: "We are now at a moment where we can prevent a famine. That's the most powerful position we could be in – to help now, rather than wait until we're in such an extreme situation that we're already losing people by the second."

Charities are urging David Cameron to announce the "biggest ever push on hunger" at his global nutrition event, to be held next week to coincide with the closing day of the Olympics. World leaders, NGOs and leading business people are expected to attend – and among other issues, it is expected that the Prime Minister will announce targets to reduce the number of under-fives– currently the figure is 180 million – who suffer from irreversible physical and mental stunting as a result of poor nutrition. More than two and a half million children die from malnutrition each year.

Barbara Stocking, Oxfam GB's chief executive, called the summit "a positive step forward", but stressed: "It must be the start of concerted action to address the shocking fact that while we produce enough food to feed everyone on the planet, about a billion will tonight go to bed hungry.

"Dwindling natural resources and the gathering pace of climate change mean that without urgent action, things will only get worse, and multiple major crises could quickly move from being an exception to being the norm."

She added that Mr Cameron should call for increased investment in small farmers, greater transparency in commodity markets and an end to biofuel subsidies.

If the world failed to listen when charities warned about the food crisis in the Horn of Africa, experts say they must pay attention when it comes to the Sahel. Six million people are already facing severe food insecurity in the region, and more than a million children are at risk of severe malnutrition.

Cycles of drought combined with low levels of agricultural investment, environmental degradation, high population growth and acute levels of poverty contribute to a context of "chronic" vulnerability, according to Oxfam. Conflict in Mali and high food prices – across the region food prices are higher by on average 25 to 50 per cent compared with the last five-year average – have exacerbated the crisis. The charity has launched an appeal and is aiming to reach 1.8 million people with emergency assistance across Senegal, Burkina Faso, Chad, Mali, Mauritania, Niger and Gambia.

Justin Forsyth, chief executive of Save the Children, said the region was in a "permanent food crisis". He added: "It is lurching from one crisis to the next. One bad year tips families over the edge, and the world responds to the emergency, but this is the tip of the iceberg. Below the surface, there is a huge ongoing crisis we don't address."

The Prime Minister told The IoS that he and Brazil's Vice President, Michel Temer, who is co-hosting next week's event, want to "use the summit to find new ways of tackling malnutrition – fostering innovation in biotechnology, encouraging stronger co-operation between governments and ensuring better accountability by Governments who receive aid".

He added that he hopes to "agree a package of measures" to "transform the lives of millions of children" before the Rio Olympics. Save the Children estimates that there will be four million more stunted children by the next Games, if current trends continue.

Unicef UK said that lack of nutrition is the "silent challenge" to global development. In 2008, eight of world's leading economists, including five Nobel laureates, ranked providing young children with micronutrients as the most cost-effective way to advance global welfare.

David Cameron added: "For every £1 spent effectively tackling malnutrition, £30 of benefit is generated."

Case studies

Assan Adaman, a 33-year-old mother of six, has lived in the same village in Kédougou, Senegal, all her life. A rice farmer, she is used to producing six bags in a good season; last year, production levels were cut in half. As she runs low on food, she is worried about the next few months and how she will ensure her children have enough to eat.

"I grew rice last year, but three bags is not enough to feed my family. I'm really worried about how I will feed them over the next few months, as we approach a bad period. I can't be calm when my children do not have enough to eat; I have to keep them healthy. We have to rely on our neighbours and our communities to help us through these harder times.

"When I receive Oxfam money, I will be able to give my children food. But I hope that the future will change for my children when they grow up."

Aissatou Kanle is a 40-year-old father of eight, living in Kédougou. He is a maize and rice farmer by trade but now works as a miner – a six-hour round trip from home – to feed his family, which he has left behind.

"Last year the maize was destroyed by floods, then rain destroyed my rice harvest. So, over the last year, I didn't have enough rice or maize to sell or for my family to eat... I had to go and find work building toilets in the mines, so I could raise money to give to my wife to provide for 10 people in my family. I had to walk three hours there and back but with very little food inside me.

"The future for my family is education. I can't just feed them without seeing them go to school, and yet I can't let them go to school without them eating, but I will fight. I don't want to see them having the same life as me."