The direct cost of actions taken by so-called "patent trolls" totalled $29bn (18.5bn) in the US in 2011, according to a study by Boston University.

It analysed the effect of intellectual rights claims made by organisations that own and license patents without producing related goods of their own.

Such bodies say they help spur on innovation by ensuring inventors are compensated for their creations.

But the study's authors said society lost more than it gained.

The research covered industry bodies that refer to themselves as "non-practising entities" (NPEs). They include businesses that buy patents with the sole aim of licensing them out, individual inventors, universities and companies that assert patent rights unrelated to the products they make.

The "direct costs" included lawyers' bills and licence fees. But authors James Bessen and Michael Meurer - both from Boston University's school of law - said other charges meant their study only reflected part of the impact.

"This [$29bn] figure does not include indirect costs to the defendants' businesses such as diversion of resources, delays in new products, and loss of market share," they wrote.