The UK government says it is activating a £2m emergency plan to tackle a cholera epidemic in the west African state of Sierra Leone.
More than 200 people have already died and more than 12,000 have been infected by the water-borne disease.
The Department for International Development (DfID) is using a new network to deliver emergency medical, water and sanitation assistance.
President Ernest Bai Koroma has declared a national emergency.
Earlier this week the authorities in the country appealed for international assistance to help them contain the spread of the disease.
It is the first time the UK government has used its Rapid Response Facility, a network that includes private businesses and specialist aid organisations, to help in a humanitarian crisis since it was established in March.
Meanwhile, charities warned it was vital to stem the outbreak before it reached its expected peak in three weeks.
Save the Children's country director for Sierra Leone, Heather Kerr, said: "If we can't get this outbreak under control quickly and comprehensively, it has the potential to kill many more children.
"Children die very quickly from cholera if they don't receive immediate medical help.
"The sheer volume of people who are contracting the disease means that aid agencies need more funding now to respond more efficiently to this devastating outbreak."
Save the Children, International Rescue Committee, Oxfam, Concern, Care International and the British Red Cross have all been mobilised as part of the response.
DfID Secretary Andrew Mitchell said the epidemic was fast becoming a crisis with millions potentially at risk.
"Not only will our response be rapid, it will be efficient. We will monitor closely to make sure every penny of British aid achieves results and supports those in dire need," he said.
"Urgent action is required to halt the spread of disease and save lives - Britain is leading the way."
DfID plans to ship anti-cholera drugs and water purification kits to Sierra Leone.
It will also supply clean water and sanitation to almost two million people, and direct treatment for some 4,500 people with the disease.
Cholera is a sort of extreme diarrhoea, which can kill people within hours if rehydration is not carried out quickly.
The United States has also offered help and organisations like the Red Cross already have resources on the ground.
The outbreak has been most severe in the capital, Freetown, where there have been 100 deaths in the last month alone. It has a dangerous mix of poor sanitation, high population density and limited health services.
Sierra Leone's health minister said the outbreak was directly linked to the country's civil war, which lasted from 1991 until 2002 and caused a massive migration from rural to urban areas.
It claimed thousands of lives as rebels from the Revolutionary United Front (RUF), supported by Liberian warlord Charles Taylor, took over the countryside and eventually seized Freetown before being ousted by British and other foreign forces.