The commercial broadcaster says National Advertising Revenue (NAR) dropped back in the three months to 30 June, after rising 12% in the first quarter.
Reluctant advertisers uncertain over the state of the economy and a World Cup inspired bounce in 2010 were blamed for the drop.
The company forecasts NAR will be down 2% in July, and 4% in August. September revenue is expected to be flat, it adds.
Despite the drop in second quarter revenue, ad receipts over the first half of the year were up 2%, behind the 3% increase for the UK TV ad market.
It does, however, still expect to outperform the advertising market for the full year.
The company claims that there are “clear signs of momentum building” in delivery of five-year transformation plan, which includes bolstering its digital offering and developing more shows it owns to sell internationally.
The commercial rights to many of the broadcaster’s ratings hits, including X-Factor and Britain’s Got Talent, are owned by production companies such as Fremantle.
ITV Studios, which makes ITV-owned shows such as Downton Abbey, saw revenue increase to £264m in the first-half, up from £254m in the same period last year.
Online revenues increased by 33% to £16m, up from £12m a year earlier.
Chief executive Adam Crozier, says that despite some progress “there is still much to be done to rebalance the business as we adapt to the digital future”.
He adds: “We’re encouraged by our progress to date and the prospects for ITV in the medium to long term, although we remain cautious about both the TV advertising market and the economy.”
Pre-tax profit for the first-half of the year increased 86% to £240m, driven the absence of costs of producing World Cup coverage.