Businesses and consumers in the eurozone became less upbeat about their prospects in August, while their British counterparts perked up as the likely impact of the U.K.'s June vote to leave the European Union was reassessed.

The changes in sentiment partly reversed those in July, when British businesses and consumers initially reacted with alarm to the Brexit vote, while their eurozone counterparts were untroubled by the unprecedented decision.

The swings in mood underline how difficult it has been to settle on a clear assessment of the vote's likely impact, given that negotiations between the U.K. and the rest of the EU on their future economic relationship have yet to start.

The near-term economic impact of the vote will, in part, depend on where sentiment settles, since a sustained weakening of confidence would likely cause businesses and consumers to cut back on spending.

The European Commission on Tuesday said its Economic Sentiment Indicator for the eurozone—which aggregates measures of consumer and business confidence—fell to 103.5 in August from 104.5 in July. That was a larger decline than the drop to 104.1 expected by economists surveyed by The Wall Street Journal. It was the lowest reading since March, but remained well above its average going back to 1990 of 100.0, indicating relative optimism among businesses and consumers.

By contrast, the U.K.'s ESI rose to 104.0 from 102.6, recouping some of the ground lost in July, while remaining well below the levels seen in the months leading up to the June 23 vote.

The decline in eurozone sentiment is one of the first indications that the prospect of a U.K. departure from the EU may weaken an already modest economic recovery. Surveys of businesses in the months since the vote have recorded a slight pickup in economic activity.

Minutes from their July meeting recorded that European Central Bank policy makers then believed the Brexit vote had created "new headwinds" for the eurozone economy, but wanted to wait until the extent of the economic disruption became clearer before deciding whether to provide additional stimulus.

They next meet on Sept. 8, but the modest drop in confidence is unlikely to give them a clear sign that the economy is set to slow.

In the U.K., measures of activity recorded a sharp slowdown in July, but other indicators since then have been more mixed, with some pointing to continued strength in household spending. The commission's measure of consumer confidence did record a slight pickup in August, although it remained well below the June level. There were also partial recoveries in sentiment among manufacturers, service providers, and retailers. However, confidence among construction companies deteriorated sharply.

Write to Paul Hannon at paul.hannon@wsj.com

(END) Dow Jones Newswires

August 30, 2016 06:05 ET (10:05 GMT)