The credit ratings agency warned that typically long-term liabilities are becoming more short-term and sometimes carry immediate funding needs.
The situation could become even more acute if further weakness in power prices speeds up nuclear plant decommissioning. "We think provisions are poised to increase to cover the costs of decommissioning works and disposing of nuclear waste," S&P Global ratings analyst, Pierre Georges, said.
It represents another headache for a sector which is already struggling with diminishing profitability and cash flow. S&P currently has six of Europe's main nuclear operators on a negative outlook, meaning they could be subject to future downgrades.
Additional pressure is also coming from recent changes to the way utilities firms' calculate the value of their liabilities.
"We believe these debt-like nuclear provisions and the sluggish environment are carving into the already tight headroom within our current ratings on European utilities with heavy nuclear exposures," Georges added.
However, utility firms could increase their financial headroom over the next few years by extending the life of existing nuclear power plants and postponing decommissioning works.
"Nuclear operators across Europe are seeking to extend the life of some of their plants, in cases where the effort is financially viable... Some of these extensions have ushered in a somewhat better financial picture," S&P said.