The research was commissioned as part of Ofcom's review of public service broadcasting to help it understand what lies in store for advertising-funded television.
Report authors Oliver & Ohlbaum Associates laid out four scenarios. The worst case foresees an 83% drop in the value of the market for TV ads broadcast in the run of scheduled programming from £3.16bn in 2007 to £520m in 2020.
The best case for linear TV ads is for the market to grow £3.43bn.
In the worst case the reason the market would be undermined is that the wide availability of user-generated and unauthorised content online would cut into TV viewing hours, limiting established broadcasters' revenue earning potential.
There would still be growth in revenues from ads seen via PVR viewing, such as Sky +, to £176m, and in on-demand TV viewing, to £651m, but this would take the TV advertising market to less than half of its current size.
The same scenario sees revenues for online display advertising (not video-based) growing from £605m in 2007 to £1.8bn in 2020 and revenues for online audio-visual advertising climbing to £453m.
In other scenarios growth in online and pay-TV services launched by broadcasters would offset the decline in the conventional broadcast ad market.