ITV told investors in March that it expected a drop of 5 per cent in April, due to an early Easter and strong comparisons with last year.
Industry sources expect the TV market is now likely to be little better than flat in the second quarter, after an estimated 2 per cent rise in the first quarter.
May is set to be up only 1-2 per cent. June should be better, but ITV may receive only a relatively modest lift from the Euro 2016 football championships.
Some advertisers began putting spend on hold last month because of the vote to remain or leave the European Union on 23 June.
Jon Horrocks, the managing partner at Blue 449, said the market may be stronger than some estimates, but admitted it has "started to come back quite a bit" and Euro 2016 "doesn’t look as strong as we originally thought".
Horrocks cut his full-year estimate for the TV market to 3 per cent, but other observers warned the figure could be closer to zero.
This view clashes with a report from the Advertising Association and Warc this week that predicted a 5 per cent increase, which is line with City expectations.
One TV source said: "Over the past three years, we have had annual growth of 5-7 per cent. At some point, there had to be a slowdown."