Britain's decision to leave the EU has led to a "dramatic deterioration" in economic activity, not seen since the aftermath of the financial crisis.
Data from Markit's Purchasing Manager's Index, or PMI, shows a fall to 47.7 in July, the lowest level since April in 2009. A reading below 50 indicates contraction.
Both manufacturing and service sectors saw a decline in output and orders.
However, exports picked up, driven by the weakening of the pound.