The figures, which cover the period 1-29 August, showed falling shares for all terrestrial channels, despite the UK population watching more TV in total.
Jim McDonald, MPG's head of broadcast, said: "Total commercial impacts were up about 4%." He added that increasing supply and falling demand was resulting in cheaper deals for advertisers.
ITV1's grip on the 16 to 34-year-old market faltered to a 13.68% share, while Channel 4 (including its +1 channel) led the category with 15.5%.
Richard Oliver, Universal McCann's managing partner, said: "ITV said last year it expected to do reasonably well this year. But it's not been the case."
An ITV spokesman said: "Share of commercial impacts is measured on an annual basis and over the course of the year it fluctuates depending on a range of factors, including scheduling, number of commercial channels and the performance of the BBC."
But Oliver said ITV1's August performance was not "out of kilter with how it performed in the year to date".
McDonald added: "ITV will be very disappointed, but it's a logical extension of programming budgets [being down] that it decided not to spend much in August."
ITV's spokesman said its autumn schedule would be "packed with hits such as The X Factor, Harry Hill's TV Burp and I'm a Celebrity... Get Me Out of Here!, plus a host of new drama".
Elsewhere, Channel 4's decline was less severe than in recent months. Its share of all-adults fell 6.2% year on year, 14% among 16-34s and 2.5% across the 35-plus market, compared with falls in July of 19%, 17% and 20% respectively.
Mike Parker, Channel 4's head of strategic sales, said: "Given that Big Brother has not had its best year and it's going in 2010, our performance has been pretty robust."
Five's performance was stable. Its share of all-adults was unchanged year on year, at 8.3%; its 16-34 share fell 9.7% - and its 35-plus share increased 0.9%.
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