The Consumer Confidence Index (CCI) from YouGov and the Centre for Economics and Business Research (Cebr) measures how optimistic people are feeling about the state of the economy.
Benchmarked at 100, above that figure consumers are generally optimistic, below 100 they are pessimistic.
The CCI fell by 4.2 points in August from 103.0 to 98.8, as inflation, largely driven by energy price rises, piles pressure on people ahead of winter.
Even measures such as house prices and job security, although still positive, fell by 7.2 points to 124.9 and 2.4 points to 118.5 respectively.
The survey was taken between 1 and 31 August, before Prime Minister Liz Truss announced her plan to freeze energy bills at £2,500 a year for a typical household.
Kay Neufeld, head of forecasting at Cebr, said: “Following a short-lived improvement in July, the 4.2-point fall in the Consumer Confidence Index in August represents the steepest decline since the early days of the COVID-19 pandemic and drags the headline index into overall pessimistic territory.