This would equate to a total loss of £1bn by 2030, the ROI specialist has calculated, based on the UK Treasury’s prediction that by 2030 the UK’s GDP would be 6.2% lower outside the EU than it would be inside it.

The damage, Zenith says, would be caused by a reduction in economic growth in the long term and not by the short-term reactions of advertisers to a vote to leave, which it predicts would be minimal. 

According to the company’s long-running Advertising Expenditure Forecasts report, over the past 35 years, the UK ad market has averaged 1.1% growth for every 1% increase in GDP. Reduced economic growth resulting from Brexit would lower the growth of the UK’s ad market in the long term, Zenith claims. 

“While the immediate effect would be muted, Brexit would have a long‐term cost for the UK ad industry, holding back its growth by £70 million a year,” said Jonathan Barnard, Zenith’s head of forecasting.

“It would also threaten to make cross‐border accounts in Europe more costly and cumbersome to operate.”