Welsh voluntary organisations could be "eradicated" if the £2.1bn they are set to lose from European Union structural funds after Brexit is not matched by the UK government.
This was among the main concerns highlighted by the Wales Civil Society Forum on Brexit, which was created this year to coordinate the voluntary sector's response to leaving the EU.
It is today due to present its findings to Mark Drakeford, Cabinet Secretary for Finance in the Welsh assembly.
Anna Nicholl, director of strategy and development at the WCVA, said: "Unsurprisingly, there are real concerns about funding and how the replacement funding, in the form of the Shared Prosperity Fund, will be shaped and delivered.
"A number of organisations and services in Wales will be deeply affected – or even eradicated – without replacement funding."
Wales was awarded £2.1bn in EU structural funds for 2014-20 to help support poorer regions. In the 2007-2013 funding round, £140.5m of contracts went to a total of 421 third sector organisations in Wales.
The UK government has yet to reveal details of the Shared Prosperity Fund.
The forum also raised concerns about post-Brexit trade agreements weakening environmental and animal welfare protections, as well as workers' rights.
"With charities, social enterprises and other third sector organisations playing an essential role in sustaining Welsh society, including the provision of vital, front-line services, the Welsh government and the UK government need to take note," said Nicholl.
Drakeford said: "Whatever form Brexit takes, it will cause disruption to Wales. That is why we, as a responsible government, will continue to plan for all possible outcomes.
"The third sector must also think carefully about how it responds to Brexit while continuing to deliver vital services to our communities.
"I welcome the forum’s position paper because it will help the third sector consider the implications in a number of key areas, such as human rights and equality, immigration, EU citizenship, the environment and animal welfare."