The latest chapter in French power group Alstom's legal battles against a former Chinese partner opened on Thursday when a Bulgarian court began examining allegations of intellectual property theft.

The alleged stealing by Chinese firms, including by computer hacking, of technologies patented by Western firms has long been an issue of contention between Brussels, Washington and Beijing as the Chinese economy has grown.

A US study compiled by high-ranking former US officials said in May that theft of software and other US-developed products was costing the American economy more than $300 billion (220 billion euros) each year -- as much as the United States sells to Asia.

In Alstom's case, it licensed in 2004 to Chinese company Insigma the right to use a technology reducing sulphur emissions in coal-fired power plants -- but only for the Chinese market.

The agreement was later terminated after Insigma stopped paying royalties, and in 2010 Alstom successfully sued Insignia at the Singapore International Arbitration Court for $35 million (25.8 million euros).

Insigma meanwhile teamed up with Italian firm Idreco and won a contract in 2008 to fit out a Bulgarian power plant -- using, according to Alstom, its technology, and beating off a rival offer from ... Alstom.

"Our major problem is the recognition that this is our technology," Alstom's marketing director for Europe Guillaume Geoffroy told AFP.

Deadline delays prompted the Bulgarian authorities to cancel the Insigma-Idreco contract in late 2012 but the installations have nevertheless entered into operation in April 2013, the plant confirmed in a statement.