Charities are calling on the UK government for greater support as they report the loss of millions of pounds as a result of Covid-19.

Oxfam said it is losing £5m a month from the closure of its shops alone, while Christian Aid last week said it expected a £6m drop in its funds this financial year. Others said they were still calculating the impact.

Bond, the UK international development network, wants the government to establish a stabilisation fund so organisations can survive and continue to deliver aid to people living in refugee camps or in extreme poverty.

Major development charities contacted by the Guardian said they expect to see a significant drop in funding because of the closure of charity shops, no face-to-face fundraising and the cancellation of key events like music festivals and the London Marathon.

Graham MacKay, Bond’s chief operating officer, said UK charities would need £4bn to cover the funding lost because of the pandemic. “At least £320m of this should be made available to charities working internationally,” he said.

The majority of the development sector will not be eligible for the £750m pot recently announced for frontline charities.

“Nobody should be left behind during this humanitarian crisis,” said MacKay, “but who is going to help get society’s most vulnerable people through this if charities collapse?

Nicola Tallett, director of engagement for Oxfam, said: “Many charities have seen their income hit hard because of the crisis. Oxfam is no exception. It’s too early to say what the overall impact will be, but the closure of the shops alone means we are losing £5m a month.”

Oxfam is furloughing about two-thirds of its UK workforce, including most shop staff.

Anticipating a drop in unrestricted income, Christian Aid is to furlough 20% of UK staff to protect country programmes and international staff. Other personnel are moving to four-day weeks, with leaders taking temporary salary cuts .

The Department for International Development (DfID) has announced a £200m package of Covid-19 support, the majority of which will go to UN agencies. A DfID spokesperson said: “We are encouraging the UN to channel the funding as quickly as possible, including to NGOs, and have allocated £20 million directly to NGOs to support them respond. They are also receiving extra funding through DfID’s new partnership with Unilever to support hygiene practices in developing countries.”

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Danny Harvey, executive director of Concern Worldwide UK, said that as well as supporting NGOs to contribute to the global response to Covid-19, DfID should show flexibility to support core costs – which can include staff and office costs – and continue to maintain funding for other projects that might have been suspended because of coronavirus restrictions. “It is vital we can continue to work and recover quickly from the effects of the pandemic in the countries where we work,” she said.

Concern said it had implemented a number of cost-saving measures, including graduated reductions to staff salaries in the UK and a voluntary option for staff to work reduced hours. The organisation said it had reduced spending on a number of activities and would draw on its reserves.

“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,” said Tallett.