The CBI said its balance for output volumes over the past three months - which measures the difference between firms reporting rising and falling output - fell to -7 in August from +6 in July, its weakest since COVID restrictions ended.
The survey's order book balance for August dropped to -7 from +8, its lowest since April 2021 and well below the median forecast of +3 in a Reuters poll of economists.
"With expectations for future growth subdued, steps will need to be taken to shore-up confidence in the short to medium term - particularly supporting vulnerable firms and consumers with energy price rises," CBI economist Alpesh Paleja said.
The CBI figures follow Purchasing Managers' Index (PMI) data published earlier on Tuesday that also showed an unexpectedly sharp decline in factory output this month.
The Bank of England has warned that Britain is likely to enter recession later this year as surging energy costs reduce households' disposable income and push up costs for businesses.
The CBI's measure of expected factory output prices over the next three months rose in August to +57 from +48, though it is below historic peaks reached earlier in the year.
(Reporting by David Milliken; Editing by William Schomberg)
