Around 5.3 million people in the African nation face starvation after a poor harvest and brutal civil war.
Members of the Disasters Emergency Committee including Christian Aid, Tearfund, CAFOD and World Vision have raised the alarm about the situation.
At least 2.8 million people are already dependent on food aid and wild foods such as water lily roots for survival.
Although a national unity government has now been formed after a two-year civil war millions of people have lost their cattle, seed stores, tools, businesses or land.
With the rainy season starting this month, vast areas will soon be cut off as most the country's extremely basic and limited roads turn into rivers of mud.
The DEC's Chief Executive Saleh Saeed said: "I have just returned from South Sudan and after years of horrific conflict our member agencies are desperate to support people to survive and rebuild their lives.
"Instead some may be forced to cut back their work due to a lack of funds.
"Communities need food, clean water and healthcare. We should be supporting people to not just survive but to return home when they feel safe to do so, to plant crops, replace livestock and restart their businesses - we won't be able to do all this without more funding.
"We are very concerned that despite some strong news coverage of the situation, public awareness of the crisis in the UK remains very low. This makes appealing for funds very difficult for our members and currently prevents the DEC from launching its own appeal."
DEC charities need £198.2m to fund their emergency work in South Sudan and to date £68.8m has yet to be secured.
