The chief executive of the National Council for Voluntary Organisations has warned that charities "must now be preparing for the consequences of no-deal".
Sir Stuart Etherington’s comments came as research from the Charity Finance Group showed that three-quarters of the sector had done little or nothing to prepare for the UK leaving the European Union without a deal.
Next week is shaping up to be a crucial one in parliament, where attempts by MPs to stave off a no-deal Brexit are expected before a suspension of parliament of up to five weeks, which was announced earlier this week.
The suspension, or prorogation, will reduce the amount of time available to MPs to prevent a no-deal Brexit, with Boris Johnson, the Prime Minister, insisting that the UK will leave the EU on 31 October with or without a deal.
On Twitter, Etherington said it was "immensely frustrating" that a no-deal Brexit was still a possibility, but charities should "deal with this sorry reality".
He said: "We continue to believe that no deal will represent a very bad outcome for the country and for charities, and again ask politicians to work to avoid it.
"In the meantime, at the NCVO we will continue to support our members in their preparation."
He added that the NCVO would publish guidance in early September on preparing for a no-deal Brexit.
In a blog, Caron Bradshaw, chief executive of the CFG, said the possibility of a no-deal departure from the EU had increased, but 75 per cent of charities had done little or nothing to prepare for such an outcome.
"It’s understandable that this is the case because we all know how detrimental to planning such high levels of uncertainty and volatility are," she said.
"But we must step up our preparation, and government needs to provide support for the sector."
The warnings come after a letter was sent to the Prime Minister, which was coordinated by the Brexit Civil Society Alliance and signed by 85 organisations, warning that charities were not ready for a no-deal Brexit and complained of a lack of support from the government for the sector.
A Third Sector article in February set out some of the areas where charities could expect to see an impact from a no-deal Brexit.
The article covers the impact on European staff, the likely constriction of funding available for charities and rising demand on their services, the potential impact of problems at the border, investments and how international charities could be especially hard-hit.