A group of UK aid agencies has launched a joint fund-raising appeal to help more than 10 million people affected by severe drought in the Horn of Africa.
Thousands of families in desperate need of food and water have trekked for days from Somalia to the Dadaab refugee camp in eastern Kenya.
The drought is the worst in East Africa for 60 years and the UN described it as a "humanitarian emergency".
A Disasters Emergency Committee (DEC) appeal will be broadcast by the BBC.
Comedian Lenny Henry will front the BBC TV appeal while broadcaster Kate Adie will voice the radio version.
The British public donated more than £1m to individual charities even before the DEC appeal was launched.
Save the Children had received £560,000, Oxfam £277,000 and the Red Cross £150,000.
DEC chief executive Brendan Gormley said thousands of destitute people were on the move into Kenya and Ethiopia.
More than 1,300 people a day were arriving in the Dadaab camp, already thought to be the world's largest with a population of 350,000.
A similar number are crossing into Ethiopia.
Many of those reaching the camps are severely malnourished children, some of whom have died soon after arriving.
Mr Gormley said: "Slowly but surely, these people have seen their lives fall apart - crops, livestock and now their homes have been taken by the drought," .
"They've been left with no alternative but to seek shelter and life-saving help elsewhere.
"We have a duty to help quickly before the situation spirals out of control."
The UK has pledged £38m ($61m) in food aid to drought-hit Ethiopia - enough to feed 1.3 million people for three months.
The DEC appeal will help people in Kenya, Somalia, Ethiopia and South Sudan, which will officially separate from the Republic of Sudan on 9 July.
Mr Gormley said: "Of course these people need a long-term solution with investment and political will - but right now it's about preventing a tragedy."
Aid agencies, including the Kenyan Red Cross, the Somali Red Crescent and Action Aid, are being helped by local groups to access remote areas with food, water and medical treatment.
UK International Development Secretary Andrew Mitchell welcomed the appeal launch.
He said: "Through no fault of its own, the Horn of Africa is experiencing a severe drought caused by the failed rains.
"The British government is already providing vital food to help 1.3 million people - but more needs to be done and we are lobbying other governments to do their bit.
"We welcome the DEC appeal to help the 10 million men, women and children caught up in the crisis.
"British charities and organisations are on the ground and ready to help, but need this additional support to get emergency supplies to those in desperate need."