ITV plc, Britain's biggest listed free-to-air broadcaster, cautioned on slowing advertising revenues in the first half of the year, heaping further pressure on European media stocks.

In a trading update published Wednesday, ITV said ad revenues for the first three months of the year fell 9% to £393 million ($509 million) and warned the first half slide could be as much as 15% to 20% thanks to the lack of a major sporting event on its broadcast calendar.

"ITV's overall performance and the shape of the UK advertising market are very much as we anticipated and our guidance for the full year remains unchanged," said outgoing CEO Adam Crozier. "The first half performance will also be impacted by the weighting of the programme budget to the first 6 months and the phasing of Studios deliveries, most significantly the non-recurring benefit of The Voice of China in 2016."

The group's forecast also appears to challenge speculation that video-on-demand revenues would shift from YouTube to traditional broadcasters in the wake of accusations that the Alphabet Inc. (GOOGL) - Get Report -owned platform was placing ads next to extremist content.

That said, ITV's online advertising revenues were up 22% in the first three months of the year, the company said, against a 32% increase in online viewership.

ITV's downbeat outlook, however, filtered through to the European media space, pushing shares in Europe's two biggest advertising firms -- Publicis Group SA (PUB) - Get Report and WPP Plc (WPPGY) -- 0.85% and 0.82% lower in Paris and London respectively. The Stoxx Europe 600 Media Index was also marked 0.53% lower at 293.23, trimming its year-to-date advance to 4%.

Last month Publicis warned that "instability in the international environment" coupled with rapid changes in technology and consumer behaviour will continue to weigh on advertising spending as clients reassessed their business models and marketing budgets.

Nevertheless, the world's third-largest advertising group said revenues for the first three months of the year shrunk 1.2% on an organic basis to €2.33 billion ($2.5 billion) but beat analysts' expectations of a decline of 1.9% and slowed the rate of loss posted in the final quarter of last year.

U.S. media groups are also finding the adjustment in viewer habits -- and ad spends -- difficult to manage. WaltDisney Co. (DIS) - Get Report said late Tuesday that revenue for the three months ending in March fell to $13.3 billion, missing the $13.45 billion average of analysts' estimates compiled by Bloomberg, even as profit exceeded estimates largely because of strong results at Disney's parks and resorts.

However, last week, CBS (CBS) - Get Report bucked an otherwise disappointing set of media earnings when it posted $3.34 billion in first quarter sales, beating an average Wall Street analyst estimate that called for $3.28 billion. Earnings also surpassed forecasts, as CBS posted $1.04 per share, compared with an expectation of 93 cents per share.

CBS's results contrasted with those of its former sister company, Viacom (VIAB) - Get Report , whose stock fell for a second consecutive day on Thursday after it reported a drop in ad sales while acknowledging that Charter Communications (CHTR) - Get Report had relegated most of its core cable networks to a higher-priced tier. Viacom, which owns MTV, Nickelodeon and Comedy Central, is likely to see a decline in affiliate fees as the majority of Charter's customers opt for the operator's standard package.