Microsoft has confirmed an April release date for a patch designed to ensure that Internet Explorer can work around a disputed patent for technology allowing web browsers to access interactive application programs.

In a blog posting, the firm warned that the ActiveX-related patch could disrupt existing uses of the technology and advised developers to test the updates on their systems. The patch has already been made available for download for this purpose, but the forthcoming April update will install the patch on all remaining systems.

The update is the result of a dispute with the University of California and tiny tech firm Eolas Technologies, both of whom sued Microsoft for patent infringement in 1999. In 2003, a jury agreed with the claim and awarded damages of $520.6m plus interest.

Microsoft appealed and, in March 2005, won a retrial. The case has been sent back to the district court, where Microsoft effectively has to show that the Eolas patent is invalid because the technology behind the patent was already in the public domain.

Its case will be made more difficult after the US Patent and Trademarks Office ruled in September 2005 that the Eolas patent was valid.

See: Microsoft's blog posting

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