Jonathan Allan, director of sales, said fears over Brexit were only partly to blame for of a slowdown in TV advertising in the second quarter.
The market will be "probably up 1% or flat" between April and June despite the Euro 2016 football tournament.
Allan pointed to supermarkets cutting ad spend in favour of price promotion and wider global, economic jitters for "softer" ad spend.
He maintained Channel 4 was still performing better than ITV, the biggest TV sales house, and had beaten the market in the first quarter.
Allan expects the TV ad market will rise only 3% this year – worse than some industry forecasters and City analysts were expecting earlier this year – and he won’t revise that view until after the Brexit vote on 23 June. "We think money may come in post-Brexit, if we stay in."
He is also pencilling TV ad growth for around 2% to 3% next year.
Commercial TV has enjoyed several years of strong ad growth, including a 7% rise last year, as brands have valued its mass broadcast reach as consumers spend more time online. But he suggested TV advertising has hit "a mid-cycle plateau".
"About quarter of it is Brexit," Allan said. "A quarter is supermarkets coming out of advertising. This isn’t just TV. The grocers are pulling out of the whole ad market and they’re putting a lot of pressure on the Procter & Gambles and L’Oreals to put money into discounting [on prices] rather than advertising. But probably half is the general global economy. It’s been quite a tough year so far, globally."
Allan was speaking as Channel 4’s unveiled its annual report that showed its sales house, which handles advertising for UKTV and BT Sport, reported record revenues of £1.17 billion, up from £1.07 billion in 2014.
He admitted that UKTV and BT Sport enjoyed better growth than Channel 4’s own family of channels – thanks in part to UKTV’s record audience share and BT’s exclusive rights to the Champions League.
"Clearly we have other partners in our sales group who are growing their audience quite quickly – with the Champions League and so on. There’s a potential degree that they’ll grow a bit faster [than Channel 4 itself]."
The Channel 4 sales house, including UKTV and BT Sport, enjoyed a 9% rise. He declined to give a figure for Channel 4’s own ad sales growth for its channels but it was thought to be around 5% last year.
Allan maintained that Channel 4’s own channels had "at least held pace with the market" and grew its advertising share, including spot ads, sponsorship and video on demand.
"I think ITV would probably have been in line with the market, Sky would probably be ahead, and Channel 5 slightly behind."
Channel 4’s portfolio of channels saw its share of all TV viewing fall from 10.9% to 10.6% last year. However, the flagship channel held steady – unlike ITV and Channel 5. "That gave us a very strong negotiating position", he said.
Channel 4 has been pushing programmatic, online video on demand and Allan expects about one third of VoD revenues to be bought programmatically in 2016. Short-form branded content is another growth area.
"We’re the most innovative sales house," he said. "We’re commercial and entrepreneurial. Channel 4 and Sky are performing well in the market versus ITV. We’re good to work with and our audiences are performing."
Channel 4’s sales house increased its share of the TV ad market from 25.9% to 26.4% last year, according to the annual report.
The market went through upheaval in 2015 when Viacom moved Channel 5’s sales into Sky Media, following a trading dispute between Channel 5 and Omnicom, and there are now only three TV sales houses in the UK.
It is thought that ITV and Channel 4 are still seeing some benefit from Omnicom clients after they stopped spending with Channel 5 as a result of the dispute. Allan would not comment.
The annual report showed Allan earned £547,000, including £164,000 bonus, and David Abraham was paid £881,000, including £188,000 bonus.