Zenith has downgraded its forecast for UK ad spend by 0.2 percentage points to 5.4%. It said growth began to slow before the UK voted to leave the European Union in June, and the result "is likely to further weaken growth over the next few years." In 2015, UK brands' ad spend grew 9.2%, and Zenith expects the growth to slow further to between 3% and 4% over the next two years.