OpenOffice.org has changed the licensing structure of its open source productivity suite.

The group previously allowed licensees to choose between the Lesser General Public Licence (LGPL) or the Sun Industry Standards Source Licence (SISSL). In the future it will use only the LGPL.

The move follows a decision by Sun Microsystems to retire the SISSL in an effort to reduce the proliferation of open source licences.

The SISSL licence was "not used much", said Simon Phipps, director of Sun's Open Source Office, on his blog.  "I don't think having it on Sun's list of preferred licences is appropriate any longer," he wrote.

Sun's decision means that it will not use the SISSL for any future projects. Existing projects governed by the licence have to decide for themselves whether they choose to follow OpenOffice.org's lead.

The changes by the OpenOffice project do not affect vendors and distributors, the organisation said, but will mean changes for developers.

Under the SISSL, developers were not required to publish the code of modifications made to the application. This changes under the LGPL.

Licence proliferation is an ongoing topic within the open source community. There are currently 58 licences officially labelled as 'open source' by the Open Source Initiative. The most common example is the General Public Licence, or GPL.

An abundance of licences could obstruct the acceptance of open source software because enterprises need to review a licence and its terms before using software.

In practice they often decide to review only a few licences and prohibit the use of open source software governed by any other licences.

Sun is the second software vendor to retire an open source licence. Intel declared that it would stop using the Intel Open Source Licence in March.

Phipps encouraged other parties to copy its example and retire outdated licences and in particular called on IBM.

"We are taking a practical step today, the first of several I hope, and committing Sun to helping with the issue [of licence proliferation]. I would encourage other companies to do the same," he said.