That was worse than all forecasts in a Reuters poll of economists that pointed to an improvement to -11.
Until recently, consumers had largely taken Brexit in their stride, helped by wages growing at the fastest pace in more than a decade and modest inflation.
That helped to support growth at a time when many companies have been cutting back on investment because of uncertainty about Brexit, but left the economy reliant on consumer spending.
"The very weak August CBI survey raises the possibility that consumers are becoming more concerned and cautious as the UK's October 31 departure date from the EU looms and expectations of a 'no deal' Brexit rise," said Howard Archer, chief economic adviser to the EY ITEM Club consultancy. Britain's economy contracted in the second quarter of the year and would enter a recession if it shrinks again in the July-September period.