The Queen is making a personal donation to a UK charity appeal for families facing starvation in East Africa.
The Buckingham Palace announcement came as Foreign Secretary Boris Johnson visited Somalia for talks about security and the drought in the region.
The government said it will match the first £5m donated by the public.
Runner Sir Mo Farah has also backed the appeal, saying it "breaks his heart" to "see children without food and water" and urged people to "act now".
The Olympic champion has been named an ambassador for Save the Children, one of 13 UK aid agencies brought together by the UK's Disasters Emergency Committee (DEC).
DEC says at least 16 million people in Somalia, Kenya, Ethiopia and South Sudan need food, water and medical treatment.
Drought and conflict are to blame for the crisis, says the DEC, which will broadcast an emergency appeal on the major television networks later.
While in Somalia, the foreign secretary met President Mohamed Abdullahi Mohamed and saw demonstrations of the training the British military is providing to Somali National Army and African Union Mission to Somalia forces.
He also helped load aid supplies from the UK on to a cargo plane.
"All of this is vital if Somalia is going to move forward to achieve long-term stability and prosperity," said Mr Johnson, who will next be visiting Uganda, Ethiopia and Kenya.
Olympian Sir Mo spent his early childhood in some of the worst affected areas in Somalia.
He said: "As a father of four, it hurts to see children without food and water, but this is a reality being faced by parents in East Africa right now.
"The drought is really bad and there are millions of children at risk of starvation."
Last month, a famine was declared in parts of South Sudan, the first to be announced in any part of the world in six years.
The government and the United Nations reported that some 100,000 people are facing starvation, with a million more on the brink of famine.
A combination of civil war and an economic collapse have been blamed.
In Kenya, the country's president Uhuru Kenyatta declared its drought a national disaster and Kenya's Red Cross says 2.7 million people face starvation.
There is also a severe drought in Somalia and Ethiopia.
'Desperate for food'
Saleh Saeed, chief executive of the DEC, said hunger was "looming" across East Africa.
He said more than 800,000 children aged under five "are now facing starvation and will die soon if we don't reach them and act quickly".
"We are hearing that families are so desperate for food that they are resorting to eating leaves to survive," he added.
Mr Saeed told BBC Radio 4's Today programme the money raised is being used to provide water, sanitation and food.
He said that, while it was important to talk about the long-term infrastructure of countries like Kenya, the areas impacted by the crisis were facing an "emergency situation".
Addressing concerns that some of the money going to the countries affected by civil war may be stolen by the government or rebel forces, he said the charities brought together by the DEC work directly with staff and vetted organisations on the ground.
Mr Saeed said it was "unfortunate" that the government of South Sudan was reportedly considering charging aid workers to enter the country.
He added: "We need to put it into perspective. The amount of money that is lost on fraud is actually quite small."
International Development Secretary Priti Patel said UK aid has funded food, water and emergency healthcare in East Africa, but more support was "urgently needed to prevent a catastrophe".
She said the international community must follow the UK's lead "to save lives and stop the famine before it becomes a stain on our collective conscience".
For more information on the DEC East Africa Crisis Appeal visit www.dec.org.uk