Sun Microsystem has stepped up the pressure on Microsoft Office by offering free downloads of the rival Office.org suite with its regular Java updates.

The suite is already freely available for download but the Sun move means it will be offered on boot-up to millions of people who have signed up to Java auto-updates.

IBM last month reported that 100,000 people downloaded its free version of the suite, brand Lotus Symphony, within a week of it being posted online.

Sun also offers a $70 (£35) version of the code under the name Star Office which includes a commercial spellchecker and thesaurus, additional fonts, templates, sample documents, and clipart graphics. An enterprise edition includes tools for centralised administration.

Microsoft's Office suite still looks unassailable but three factors could enable rivals to erode its market share. One is the fact that it costs a lot of money, and its open-source rivals are free.

Secondly, companies deciding whether to adopt the new and improved Open XML formats in Office 2007 may take the opportunity to look at rival products.

Thirdly, the OpenOffice.org suite in its various guises supports the ODF formats which have been adopted as an international ISO standard. Some government organisations have said they will only use document formats conforming to an official international standard.

Microsoft' bid to get Open XML adopted as an ISO standard has hit problems though it is expected eventually to succeed.