The letter, which is addressed to Rishi Sunak and has over 240 signatures from British politicians, asks the chancellor to take "urgent action" to support charities, voluntary organisations and social enterprises.
Labour's Stephen Doughty, the MP behind the letter, told ITV News he had spoken to several charities who are worried the coronavirus lockdown could mean they'll never open again.
He pointed out how "a lot of people are being very generous" through this "unprecedented situation" but said "it's a question of billions of pounds suddenly not appearing".
He said despite many people still giving to charity, many organisations rely entirely on events, such as the now-cancelled London Marathon, and payments will be due, but no money will come in.
The letter says "without an immediate injection of money, many charities, voluntary organisations and social enterprises of all sizes will soon close. Funds are running out".
The letter demands:
"Immediate emergency funding" for frontline charities helping in the battle against Covid-19.
A "stabilisation fund" to be provided to all charities to help them stay afloat during the pandemic
Confirmation that all charities, voluntary organisations and social enterprises are eligible for government action that pays up to 80% of salaries for retained workers
Confirmation that charities, voluntary organisations and social enterprises eligible for the business interruption loan scheme provided to businesses
Confirmation that the relaxation of business rates applies to premises that are not charity retail shops
The government to work with philanthropic organisations, such as the National Lottery, to identify grants that can be provided "early" to fill the "growing gaps" in the sector
The National Council for Voluntary Organisations (NVCO), predicts charities will miss out on a minimum of £4.3bn of income in the next 12 weeks.
It says "every day counts" in the battle to win support from the government and claims charities are "facing imminent collapse" if additional funding isn't provided.
Karl Wilding, chief executive of the NCVO said: "Every day counts here. I’m hearing from charities whose income has disappeared overnight but who still have to run services for their communities.
"Many of them have very little emergency cash to tide them over, and even those that do will run out in a matter of weeks."
He added: "I know the government is working on this but for many charities around the country there is very little time to spare."
In an attempt to help the industry, Martin Lewis, founder of Moneysavingexpert.com, released £1million from his personal charity fund, to provide grants of £5,000 to £20,000 to "small registered charities, or local arms of bigger charities, across the UK – to help with specific UK coronavirus-related poverty relief projects".
Mr Doughty, MP for South and Penarth, said it would be "absolutely disastrous" for the industry if charities were forced to close, but said he's been in touch with the charities minister who is working to provide support.
The former head of Oxfam in Wales said: "Discussions are ongoing with representatives of the charity sector but the urgency of this cannot be underestimated."
"Every single day that's going by without some sort of solution coming forward is potentially putting them at huge risk."
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