The media agency explained that Brexit would lead to a drop in economic growth in the long term, which would cause adspend to fall.
The report added that advertisers’ short-term reaction to leaving the EU is "likely to be minimal".
It comes on the same day that PwC reports the UK entertainment and media sector is set to be the largest in EMEA by next year. The study put the growth down to rising ad revenues.
According to PwC, the UK entertainment and media sector will be worth £68.2bn in 2020, compared with £61.3bn for Germany.
Zenith has based its estimations on the Treasury’s findings that UK GDP would be 6.2% lower if outside of the EU.
The report said: "Zenith has tracked a clear relationship between economic growth and the health of the advertising industry. Increased growth leads to more products from existing advertisers and more new products from start-ups, all of which need to be marketed to consumers.
"Over the past 35 years, the UK ad market has averaged 1.1% growth for every 1% increase in GDP, according to Zenith’s long-running Advertising Expenditure Forecasts report. Reduced economic growth resulting from Brexit would lower the growth of the UK’s ad market in the long term."
Zenith added that Brexit would not have an immediate impact on UK advertising because brands are "generally pragmatic" in setting budgets. However Zenith expects budgets to change once the impact of the vote to leave is clearer.