The Landmark system, which agencies rely upon to supply Barb with Sky's audience data, has been encountering problems since its installation at the beginning of September. ITV uses the same system.
Jim McDonald, head of broadcast at MPG, said: "It is a problem for people in my position who are making judgements about the TV market. Right now, we have hardly any data coming through from Sky and it looks as if the broadcaster is performing worse than it is."
He added: "If Sky doesn't do anything about these problems, it would logically mean that TV buyers would give them a lesser share of their budget, because we make our judgements on the share of audience data."
Sky's head of trading John Litster confirmed that the broadcaster's new sales and bookings system is still experiencing "teething issue" problems.
"They are nearly all sorted and we should be up to speed with impacts within the next couple of weeks," he added.
Litster declined to comment when asked why the old system wasn't kept running in tandem in order to avoid any disruption.
Five's sales director, Kelly Williams, commented: "The fault is a technical one, which should be very fixable - but it hasn't been yet and we're not sure why.
"The problem is that without the Sky data, no one can measure any other channel's share of commercial impacts."
Phil Hall, associate director of TV at MediaCom, added: "At the moment, we can't analyse Sky's performance because we have no figures. Therefore, we can't figure out any channel's share of audience. This makes it hard to analyse the current TV market."
Litster added: "We changed systems because our old one was 10 years old and we needed to modernise.
"And we aren't the only ones to change recently - Viacom Brand Solutions has done so also."
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