News Corporation may have to write off over $1 billion (€1.02bn; £0.66bn) after its Gemstar-TV Guide unit lost a legal battle to protect its patents.

Gemstar, a provider of on-screen television listings, is proving a troublesome asset for the Murdoch empire, which has already written down its investment in the unit by $4.2bn this year.

The latest blow to NewsCorp’s bottom line follows a ruling by the US International Trade Commission that Gemstar’s patents, though valid, could not be used against three importers of set-top boxes into America. The unit had been hoping to exploit its intellectual property to charge rival TV guides and improve its share price.

However, a further drop in Gemstar’s stock could give Rupert Murdoch an opportunity to take control from founder Henry Yuen.

Under an agreement forged two years ago, the division will be managed by Yuen until 2005, though his tenure at the top may be curtailed if Gemstar continues to under-perform.

Data sourced from: Financial Times; additional content by WARC staff