Customers using Opera's mobile browsers will now see a Google search bar on their start pages, after Yahoo pulled out of a previous agreement.

Google is the new default search engine on both the full Opera Mobile (for smartphones) and the scaled-down, downloadable Opera Mini (for lower-end phones).

Yahoo said it had decided to call off its year-old deal with Opera, but did not give reasons.

"Consumers with Opera browsers will continue to have access to Yahoo oneSearch, and as long-standing partners, Opera and Yahoo will continue to work together," Yahoo continued. OneSearch is specifically designed for mobile users.

While Google has been the default option on Opera's desktop browser for seven years, the mobile browser deal is new.

Opera Mobile has shipped on 100 million phones from manufacturers including Motorola, Sony Ericsson and HTC, the browser company claims.

Meanwhile, more than 35 million people have downloaded Opera Mini, which communicates with backend servers operated by Opera, or in some cases a mobile operator. Opera Mini users browse more than 1.7 billion webpages each month. Much of that traffic comes from the search function, Opera says.

The Google-Opera announcement is another indication of the competition among search providers for a foothold in the mobile market. Google and Yahoo have each wracked up wins recently. The search providers hope to find a new and potentially significant revenue stream from mobile advertising as an increasing number of mobile users access the Internet from their devices.

Nokia recently announced that it will feature Google search on some of its phones. Yahoo also recently recorded a significant win, by replacing Google as the preferred mobile search provider for T-Mobile in Europe. Yahoo also powers search for AT&T.