Too many new media companies are chasing too little advertising revenue, one of the advertising industry's most senior executives has warned, as fresh questions were raised about the fast-rising valuations of 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.”

Speaking at this weekend's Monaco Media Forum, Mr Lévy likened the boom in businesses whose revenue models depended on the continued growth of online advertising to the dotcom boom and bust that began a decade ago.