Microsoft will tomorrow signal a direct assault on one of the main profit-generators of Google and Yahoo when it unveils an advertising service that lets users link their messages to the results of internet searches.

Though widely expected, confirmation of the software giant's plan to build its own advertising network around its search engine will intensify competition in the fastest-growing corner of the online advertising market. Microsoft officially launched its own search engine this year, although it has continued to rely on Yahoo to sell advertising on the service.