Oxfam says that it is losing £5 million a month because of the closure of its shops alone, and has had to furlough two-thirds of its UK workforce.
"Now, more than ever, support is needed to help prevent the spread of coronavirus in vulnerable and poor communities already suffering due to conflict, disaster, and climate change, and to help them cope with growing risks of hunger and even greater poverty due to the pandemic," Nicola Tallett, director of engagement for Oxfam, told the Guardian.
Graham McKay, chief operating officer of the UK international development network Bond, told the Guardian that UK charities would need £4 billion to cover lost funding so that they can continue to function, including providing aid to refugee camps and people living in extreme poverty.
"At least £320m of this [funding] should be made available to charities working internationally," he said.
"Nobody should be left behind during this humanitarian crisis, but who is going to help get society’s most vulnerable people through this if charities collapse?" he added.
The UK government has announced that it will provide charities across the country with £750 million in funding.
The charity Christian Aid has also said that it expects a £6 million drop in funds this financial year, and is set to furlough 20% of its UK staff.
Oxfam previously warned that the pandemic could take millions of lives in low- and middle-income countries, and called for an emergency aid package of $160 billion.
People living in extreme poverty often lack access to clean water and health care, and for those living in slums or refugee camps, social distancing is nearly impossible, leaving them especially vulnerable to the spread of COVID-19.