Internet giant Yahoo! Incorporated Is fortifying its free e-mail service with 25 times more storage and freeing up millions of previously claimed e-mail addresses in an effort to thwart a looming threat from its increasingly disruptive rival Google Incorporated.

Beginning Tuesday, all of Yahoo’s free e-mail accounts will be upgraded to 100 megabytes (MB), a move spurred by Google’s plans to offer 1,000 MB of free storage through its Gmail service, which has remained in a test phase since early April.

Yahoo has been offering four megabytes of free e-mail storage, although some people with accounts opened several years ago have six MB of free storage.

Sunnyvale-based Yahoo disclosed that it would be increasing its free storage to 100 megabytes during an analyst meeting held last month, but hadn’t provided a specific time for the upgrade until now.

The company hopes to appeal to e-mailers in other ways, too.