The grim news -- the first indication of economic contraction since Britain's shock EU exit vote -- was revealed at the end of a week in which Prime Minister Theresa May toured France and Germany to soothe Brexit jitters.
Private sector business activity, as measured by research group Markit's Purchasing Managers Index (PMI), sank in July to 47.7 points.
It was the lowest level since April 2009 following the global financial crisis, and sparked predictions from some quarters of a painful recession.
"Our economy has suffered a shock from the referendum vote... That has had an impact on confidence," Britain's new finance minister Philip Hammond said in an interview broadcast on Sky News.
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"Our job is to try and restore as much certainty as we can as quickly as we can," he said.
The survey -- regarded as a key early indicator of economic activity before official statistics are published -- comes after Britons voted to leave the European Union in a crunch referendum on June 23.
"The downturn, whether manifesting itself in order book cancellations, a lack of new orders or the postponement or halting of projects, was most commonly attributed in one way or another to Brexit."
Markit said the survey signalled a 0.4-per cent contraction in the British economy in the third quarter or three months to September.
"Much of course depends on whether we see a further deterioration in August or if July represents a shock-induced nadir," Williamson added.
The sharp contraction was triggered by falling output and orders for the first time since the end of 2012, while business optimism in the services sector hit a seven-and-a-half-year low.