The report on business trends from the accountancy and services group BDO said the British economy was already showing signs of slowing ahead of the EU vote on June 24, BBC reported on Monday.
BDO head Peter Hemington said uncertainty prompted by the Brexit had also disrupted investment in the UK economy, BBC said citing the report.
"We are at a crucial moment where we must be sensible in protecting the UK economy. We need a plan of action now that gives businesses the added confidence to progress with investment plans," Hemington said.
The report showed business output, which reflected company orders for the next quarter, slipped to 99 last month, compared with 99.7 in May and 100.6 in April.
Meanwhile, business optimism also slid, falling to 98.9 in June from 99.4 in May.