Too many new media companies are chasing too few advertising dollars, one of the advertising industry’s most senior executives has warned, as fresh questions were raised about the fast-rising valuations of prominent online groups.

“Everyone is seeing advertising as the manna,” said Maurice Lévy, chairman and chief executive of Publicis, one of the world’s largest marketing groups. “Far too many people are building plans based on advertising and they may well be disappointed because there is not enough money for everyone.”