Knowledge Ecology International, a non-profit public interest body, the UK's Open Rights Group, which was founded in 2005 by 1000 digital activists, and Stallman, sent the letter to Neelie Kroes, the European Commissioner's competition commissioner, on October 19.
"Oracle seeks to acquire MySQL to prevent further erosion of its share of the market for database software licenses and services, and to protect the high prices now charged for its proprietary database software licenses and services," the letter said.
"If Oracle is allowed to acquire MySQL, it will predictably limit the development of the functionality and performance of the MySQL software platform, leading to profound harm to those who use MySQL software to power applications."
The competition regulators are not happy that a company which owns the predominant proprietary database should also own the open source database with the biggest marketshare and want to assure themselves that the development of MySQL will continue apace.
The letter praises the dual-licensing of MySQL which, when it was both an independent company and after its acquisition by Sun Microsystems, was available both as a free download and for payment. The paid version had a few proprietary add-ons.
"With excellent management and considerable trust within the user community, MySQL became the gold standard for web based FLOSS database applications," the letter sent to Kroes said.
Stallman and the two organisations have no objection to Oracle's acquisition of the rest of Sun. They say Oracle will be more inclined to focus on developing its own proprietary database and not pay attention to MySQL.
The letter also points that, given the absence of the words "any later version" in the GPL version 2 licence that accompanies MySQL, it will be difficult to combine code from MySQL with any projects covered by the GPL version 3.
Hence, forking in the future would be dependent on contributions from the community, something which is not seen as happening if Oracle acquires MySQL.
"As only the original rights holder can sell commercial licenses, no new forked version of the code will have the ability to practice the parallel licensing approach, and will not easily generate the resources to support continued development of the MySQL platform," the letter said.