More than a quarter of a million people are estimated to have died during the recent famine and food crisis in Somalia, and more than half were children under five, making it the worst famine in the past 25 years, according to figures published on Thursday (pdf).

A study, commissioned and funded by the Food and Agriculture Organisation's (FAO) food security and nutrition analysis unit for Somalia (FSNAU) and the famine early warning systems network (Fewsnet), estimated that 258,000 people died in southern and central parts of Somalia between October 2010 and April 2012, including 133,000 children under the age of five.

The figure is significantly higher than the death toll from the country's 1992 famine, during which an estimated 220,000 people died over 12 months, although this famine was considered more severe because a higher percentage of the population died.

The study suggests that 4.6% of the total population and 10% of children under five died in southern and central Somalia over the crisis period. Lower Shabelle, Mogadishu and the Bay area were hardest hit, with the number of child deaths estimated to have reached 18%, 17% and 13% respectively.

Death rates peaked at 30,000 a month between May and August 2011. The UN declared famine in two regions in Somalia – Bakool and Lower Shabelle – in July 2011, following repeated warnings of an impending crisis after severe drought and failed harvests. By August, the UN said three more regions were in a state of famine (video) – Middle Shabelle, Afgoye and parts of Mogadishu. Other parts of Somalia, particularly the south, were in the grip of a severe food crisis by this time, with high rates of acute malnutrition.

The drought, the worst in the region for 60 years, led to livestock deaths, reduced harvests, and drops in labour demand and household incomes. Poor harvests drove food prices to extreme levels, said the report. The situation was compounded by conflict and insecurity in Somalia, which impeded the delivery of food aid. The Islamist group al-Shabaab was at war with the government, and areas the group controlled were some of those worst affected by the crisis.

The level of humanitarian assistance to Somalia had decreased between 2010 and 2011. Donors have been criticised for their slow response to the crisis. Fewsnet and the FSNAU first warned of severe drought across the region in August 2010 and, in March 2011, said famine was likely if the April to June rains failed. They did.

However, major appeals for donations launched in September and October, a drop in food prices and a successful rainy season meant that by November the situation had improved. The UN declared an end to famine in February 2012.

The FSNAU/Fewsnet study covered all of southern and central Somalia, the areas most affected by famine and food insecurity between 2010 and 2011. It also assessed mortality rates among new Somali arrivals at Dadaab refugee camps in Kenya and at Dollo Ado in Ethiopia.

Researchers drew on more than 200 mortality surveys carried out by the FSNAU since 2007, including 61 from the emergency period, along with data on food prices, wages, conflict, humanitarian assistance and epidemics.

The lead authors, Francesco Checchi, from the London School of Hygiene & Tropical Medicine in London, and Courtland Robinson, from Johns Hopkins Bloomberg School of Public Health in the US, concluded that though their study was not designed to investigate the cause of the high death rates, "nevertheless, an obvious rational use of our estimates would be to ensure that in Somalia and food security-related emergencies elsewhere, future early warning and surveillance alerts do translate into tangible, immediate relief interventions to support livelihoods, health and nutrition in the affected communities."

Chris Hillbruner, decision support adviser for Fewsnet, said: "Estimating mortality in emergencies is an imprecise science, but given the quantity and quality of data that were available, we are confident in the strength of the study. It suggests that what occurred in Somalia was one of the worst famines in the last 25 years."

Senait Gebregziabher, country director of Oxfam Somalia, said the number of deaths could, and should, have been prevented. "Such a shocking death toll must never be allowed to happen again," she said.

"Famines are not natural phenomena, they are catastrophic political failures. The world was too slow to respond to stark warnings of drought, exacerbated by conflict, in Somalia, and people paid with their lives."

Ben Foot, Somalia country director for Save the Children, said: "While conditions in Somalia have improved in recent months, the country still has one of the highest rates of child malnutrition and infant mortality in the world."

A Dangerous Delay (pdf), a report published last year by Save the Children and Oxfam, showed how the international community's slow response to the famine cost tens of thousands of lives.

On Tuesday, a conference in London will discuss Somalia's future, with the aim to provide international support for the government to rebuild the country.

Gebregziabher said the conference must invest in the country's long-term development, creating jobs, supporting farmers and pastoralists and ensuring trained, accountable security forces.

"It is crucial that women and men from across Somalia are involved in a bottom-up process to determine the country's future. Top down 'solutions' don't work. The country is crying out for just and sustainable peace, and the new government must grab this moment to secure it," she said.