Chief executive Mr Duncan, who is quitting after five years at C4, said advertising regulation was still stuck in pre-internet days.
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He told TV movers and shakers at the Royal Television Society's Cambridge Convention: "This is a massively over-regulated industry, especially in the advertising area. It is barmy, it's absolutely barmy."
He continued: "The TV broadcaster is stuck in a system based on 20, 30 years ago, when all the advertising (money) was on television and there weren't any competitors. The internet has to be thought about alongside television.
"The change we're going through is going to accelerate and we have a regulatory policy and a debating cycle which is years behind."
He warned: "If I have one big point to make, it is that this whole industry I think is in denial about the scale of change that is coming.
"And if we don't get de-regulation, more freedom around advertising, the internet companies, the West Coast and America, will suck more and more money out of British content, and we'll end up with a weaker system.
"And it's a really big problem."
Mr Duncan joined Mark Thompson, director-general of the BBC, ITV executive chairman Michael Grade and Dawn Airey, chair and chief executive of Five, to discuss the major issues facing broadcasters as the convention wound up today.
Mr Grade echoed Mr Duncan's view that potential revenue from UK content could end up in the US.
He hit out at the Competition Commission's decision to block Project Kangaroo, a plan by BBC Worldwide, ITV and Channel 4 to launch an on-demand video service.
He said that because of that decision "much of the money that our content would generate will end up in America".
He added: "What we need now is decisions. There is a point now where any decision is a good decision, we just need the decision.
"Every time someone comes up with a brilliant idea we go into another round of consultations, by which time there's an election, there's a whole new lot in, and we start all over again."
Culture Secretary Ben Bradshaw has said he wants to change the Government's approach to product placement, allowing it on television for the first time.
Ministers hope lifting the ban will throw a lifeline to struggling independent broadcasters such as ITV by opening new multimillion-pound revenue streams.
Ofcom's chief executive Ed Richards said the Government's decision to examine the issue of product placement opened up the way for Ofcom to look at sponsorship "so there's quite a significant opportunity there".
Much of the conference has been dominated by a fiery debate prompted by Mr Bradshaw's calls for the BBC Trust to be scrapped and the corporation to stop expanding.
Mr Bradshaw gave a keynote speech at the RTS event on Wednesday and afterwards became embroiled in a heated exchange with BBC Trust chairman Sir Michael Lyons.
Yesterday, Mr Thompson hit back at Mr Bradshaw's questioning of the Trust as both "cheerleader" and regulator.
Mr Thompson said: "The people Ben should ask this question of is those colleagues of his in the present cabinet who invented the BBC Trust, approved it and enshrined it in a charter which still has well over seven more years to run."