The Department for International Development (DFID) says both countries are facing a “real threat of famine” – with millions of people going hungry.
It is hoped the funding for food, water and emergency healthcare will help to save more than one million lives.
International Development Secretary Priti Patel has warned there is also a credible risk of famines in Yemen and North East Nigeria – with conflict and drought in some parts of Africa creating a “series of unprecedented humanitarian crises”.
She said: “Our commitment to UK aid means that when people are at risk of dying from drought and disaster, we have the tools and expertise to avoid catastrophe.
“While we step up our support for emergency food, water and life-saving care to those in need, our message to the world is clear: we must act now to help innocent people who are starving to death.”
According to DFID, more than six million people in Somalia have no reliable access to food – and 360,000 children are acutely malnourished.
There are fears Somalia is going to face a famine as bad – or worse – than one in 2011 which killed an estimated 260,000 people.
DFID said the UK’s aid package for Somalia will provide emergency food and safe drinking water for up to a million people, as well as nutritional support for more than 600,000 starving children, pregnant women and breastfeeding mothers.
The department explained that famine has been declared in several parts of South Sudan, and more than half of the population is now in desperate need.
Almost five million people face the daily threat of going without enough food and water, and three million people have been forced to flee their homes because of violence and widespread rape.
Mike Noyes, who works for the poverty charity ActionAid, described the UK’s contribution to South Sudan and Somalia as “vital”.
He added: “The need is huge and countless lives are at stake. Today, in parts of Africa, children are dying of hunger whilst their mothers watch in despair.
“The world cannot stand by and let this continue. The Government is right to be sounding the alarm.”
Skynews