The Basic Payment Scheme is 'crucial' to Welsh farming businesses and ministers should maintain current levels of funding, NFU Cymru has warned.
Industry concerns have been raised following the recent UK spending review, which showed a funding shortfall of £95m for Welsh farming.
The union has written to Rural Affairs Minister Lesley Griffiths asking her to commit to continuing current levels of funding for BPS.
The EU's level of support has been described as a 'key safety net' for Welsh farmers and rural communities.
In his letter to the minister, President John Davies said: “The BPS is more important than it has ever been, providing stability to Welsh farming in the face of dealing with a pandemic that has caused major disruption to food supply systems.
"Whilst farmers have shown remarkable resilience in keeping the nation fed throughout the pandemic, the longer-term impact of this pandemic on our economy and food supply chains is, as yet, unknown."
He added: "The BPS has been, and will continue to be, crucial if Welsh farmers are to remain able to secure the supply of safe, high quality and affordable food to all in society.
"It is a key safety net for Welsh farmers, with on average more than 80% of farming income in Wales being reliant on the BPS, a situation not unique to Wales."
The 2021-2022 Welsh agricultural budget will be £242 million instead of £337 million - a cut of around 28 percent, Chancellor Rishi Sunak confirmed last week.
The review came just days after the three devolved governments of Wales, Scotland and NI wrote jointly to Defra Secretary George Eustice urging him to provide assurances that the budget for agriculture would be maintained.
Farmers are just weeks away from potential disruption to markets as the end of the transition period nears, alongside the upheaval that the pandemic has caused to the UK food supply chain.
National Sheep Association Cymru said the funding shortfall made it 'very difficult' for Welsh farmers to see a 'way forward'.
“NSA Cymru understands Westminster are claiming the devolved nations will have this money from the EU but it is previously allocated money and not for future farm payment schemes," Chair Kate Hovers said.
“We are constantly told to be resilient and plan for the future but when we have no control over the price of our end product and then suddenly see a reduction in promised funds, this makes it very difficult to see a way forward.”