The YouGov/Centre for Economics and Business Research business confidence index, which measures businesses’ outlook, was at 112.6 in a survey just before the referendum but fell to 105.0 in research carried out after the result.
It comes after last week's consumer confidence survey by YouGov/Cebr, which was at its lowest level since 2013.
The latest poll of 1,500 business-decision makers said the share of companies that are pessimistic about the economic outlook over the next 12 months doubled to 49% from 25% in the course of a week.
Businesses are also more gloomy about their own company’s outlook, with pessimistic sentiment moving from 16% before the referendum to 26% after it.
Expectations for domestic sales have also fallen by almost 14 points in a week from 118.8 to 104.9, while hopes for exports dropped by 15 points (115.3 to 99.8). Hopes for investment also dropped by eight points (108.0 to 100.1).
Scott Corfe, director at the Cebr, said: "So far the only evidence we have for how the economy is in the wake of the Brexit vote comes from the markets which are showing a mixed picture. These figures show what is happening on the ground and they suggest a significant shock reaction.
"Not only are businesses feeling much more pessimistic in general about the state of the economy, but their own expectations for domestic sales, exports and investments over the next 12 months have gone off a cliff. Hopefully this is a short term panic reaction and confidence will edge up again once the dust settles."