Sir Stuart Etherington, chief executive of the National Council for Voluntary Organisations, has called on charities to speak up about the "very real consequences" of a no-deal Brexit scenario to avoid further harm to communities.
In a new-year message for the voluntary sector, Etherington says charities should be considering what will happen after Brexit, the sort of country we want to live in and how the sector can help to shape it.
He says many charities, including the NCVO, have not taken sides in the Brexit debate, preferring to inform people about the implications of the different ways forward.
"But it is important that charities can shape the debate about the country we will be after Brexit, and to address the issues that undoubtedly contributed to people voting to leave," he says.
"For me, that includes outlining the very real consequences of a no-deal scenario. We must also use our voice to call on politicians to act in the national interest and avoid further harm to our communities and their futures.
"Time is running out for the government to get a deal that can secure the approval of parliament. No-deal Brexit will have serious implications for our society."
Etherington says that the voluntary sector will "no doubt play a role in dealing with problems caused by Brexit".
He says: "If a crisis emerges, we will be there, just as we have in the past. But we need to do more than offer a diagnosis.
"We have to answer the questions above about what happens after March 2019. And there may be less time than we think, particularly if political impasse precipitates a general election this year."
He says this means charities need a vision for the future, "rooted in an optimism shaped by our ability to change life for the better".
Etherington says: "We need a new narrative, rooted in the traditions of what is best about civil society: community, kindness, equality, respect, aspiration and above all an impulse to help.
"And a new agenda – where civil society can help define the sort of country we want to be."
He adds: "We are not the country we were before the referendum, but neither are we the country we shall become. There is real opportunity to change the kind of country we will be in future.
"Brexit presents an opportunity to reflect on the major challenges we face as a country and how, we as civil society, can help to address them."