Mobile search is gaining in both popularity and frequency of use in the United States and Western Europe, according to comScore M:Metrics.

The research firm reports that in June 2008, 20.8 million U.S. mobile subscribers and 4.5 million European mobile phone subscribers accessed search during the month, an increase of 68% and 38% from June 2007, respectively. The United Kingdom had the highest penetration of mobile subscribers using search at 9.5%, followed closely by the United States at 9.2 percent.

The number of people accessing mobile search at least once a week grew 50% in Europe, with France and Spain leading at a rate of 69% and 63%, respectively, according to comScore analyst Alistair Hill. The number of U.S. users accessing mobile search more than doubled as a result of expanded 3G penetration and smartphone adoption, as well as the proliferation of flat-rate data plans.

Google is the preferred brand for browser-based searches with a 60% share of mobile searchers in all countries measured by comScore M:Metrics. Yahoo! ranks second in Germany, Italy, the United Kingdom and the United States. In the United States, Yahoo’s  mobile searcher penetration is 34.6%, more than double its share in most other countries.