"We find that online is the most efficient channel, but when creating a big channel mix it doesn't sit naturally next to traditional media," said Rhodes. "You can't buy a certain audience and reach as you can with TV and impressions aren't a traditional way of measuring, so it's hard to measure it against TV or print."

Matt Simpson, group head of digital at OMD and IPA digital chair, said, "In direct response, the case has been made strongly for online's accountability. But for less direct response we're lacking the proof, rather than online not doing anything. A lack of currency doesn't help."

Despite the lack of transparency, 61% said they intend to spend more than half their marketing budgets in the coming year on online marketing, with investment shrinking in traditional channels.

The survey also found 92% think social networking is the least effective online marketing medium.

The research also found that efficiency was hindered by technology issues, with 40% admitting their campaigns ran into technology problems. Only 48% admitted to always ensuring that a website can cope with the increased traffic during a marketing campaign.