By Jason Douglas 

LONDON--A survey of British manufacturing firms in July paints a mixed picture of the sector's prospects in the immediate aftermath of Britons' vote to leave the European Union, adding to signs that the road ahead may be bumpy for the U.K. economy.

The Confederation of British Industry, an employers' lobby group, said its regular survey of sentiment in the sector showed the fastest decline in optimism among manufacturing executives since 2009, reflected in a sharp pullback in investment intentions. But exports are expected to rise on the back of a falling pound.

The findings add to signs the economy is at risk of slowing, but the scale and duration of any slowdown won't be known until firm data begins to trickle out in the fall.

Rain Newton-Smith, the CBI'S chief economist, said the government needs to do more to dispel the uncertainty surrounding the economy's prospects now the U.K. has voted to leave the EU. Voters chose to quit by 52% to 48% in the June 23 referendum.

"It's important now for the new government to steady the ship with a plan, and a clear timetable, for negotiating the U.K.'s relationship with the EU. This, along with a renewed focus on industrial strategy, will help give firms the confidence they need to grow and create jobs," she said.

The CBI said the balance of firms expecting a pickup in orders over the next three months fell to zero in July, its lowest level since January 2012. The balance reflects the percentage of respondents expecting an improvement versus those anticipating a decline. July's reading followed a reading of +14% in April.

The balance expecting an improvement in exports was +10%, down from +20% in April but still signaling healthy growth. Firms credited the falling pound with supporting export expectations, but they warned they expect it to push up costs, and ultimately prices for consumers.

The CBI's findings echo those of other surveys conducted in the aftermath of the vote. The CBI quizzed more than 500 firms between June 27 and July 13.

A survey of purchasing managers in services and manufacturing published by financial information group IHS Markit Ltd. Friday found activity fell in July to its lowest level since 2009, a signal the wider economy likely slowed. Separately, the Bank of England last week said its network of regional agents reported a marked increase in perceptions of uncertainty among businesses, with some saying they were reconsidering investment and hiring plans for the year ahead, but firms said they didn't intend to cut spending and investment in the near term.

Write to Jason Douglas at jason.douglas@wsj.com

(END) Dow Jones Newswires

July 25, 2016 11:39 ET (15:39 GMT)