Airlines and financial services are also expected to cut their marketing after sterling and share prices slumped. EasyJet and Foxtons have already issued post-Brexit profit warnings.
Adrian Coleman, group chief executive of VCCP Partnership, said he expected the UK ad market to fall "between 5% and 10%" in the next six months. He is already budgeting for a 5% drop in fee income for the full year.
James Murphy, chief executive of Adam & Eve/DDB, said: "We took a call at 3pm last Friday from a US client, who pulled a £1.5m-fee project. The most worrying thing is how angry and disappointed he was."
The Advertising Association wrote on Monday to business secretary Sajid Javid to urge him to bolster "advertiser confidence" as a "priority" and push for further deregulation.
Global stock markets suffered a record $3trn two-day fall after the vote but media industry sources said "it does not feel like 2008 yet". Most brands have not altered their marketing plans and have already committed to spend in the third quarter of the year. "The challenge is what happens in 2017 and 2018," one ad sales source said.
Jon Goldstone, a former Unilever marketer and now a partner at coaching network The Brandgym, said a weak pound would disproportionately affect brands that use imported raw materials. "When this has an impact on the P&L, it’s the marketing investment that has to give," he said.