Enders expects a 3% recession in the UK advertising market if the UK leaves the European Union without a trade deal this year.
The warning compares with a prediction of 2.7% growth for the ad market if there is an orderly Brexit, with Enders predicting that to contribute to 1.5% growth in UK GDP for 2019.
Prime minister Theresa May has proposed a Brexit withdrawal deal that MPs are set to vote on in parliament tomorrow.
A 3% ad recession would be smaller than the contraction in 2009, when total adspend plunged 13% amid the global financial crisis.
Even if a "no deal" Brexit is averted, Enders is predicting the TV ad market to decline by 2.9% this year to £4.98bn. TV spend growth is estimated to be flat for 2018 (up 0.3%), after shrinking by 3.2% in 2017.
TV ad revenue would be particularly hit by a "no deal" Brexit, Enders explained, with advertisers either reducing adspend or shifting part of their TV budgets to other media.
Assuming an orderly Brexit, Enders is forecasting pure-play online media spend to grow by 8.3% this year. This is significantly weaker than in previous years, with an estimated 11.6% in 2018 and 15% in 2017.
The report added: "The total advertising figures partly mask the pressure on UK consumers, due to an expansion of the measured advertising spend universe. This is due to significant self-serve online advertising growth by SMEs and non-advertising marketing budgets moving to online advertising platforms."