After already cancelling the launch of her Last Girl On Earth Tour with Ke$ha, Rihanna has now been forced to scrap another two shows on her U.S. tour, due to poor ticket sales.

Her tour promoters, Live Nation, have cited 'production changes' and 'scheduling conflicts' as the reason.

The cancellation follows news that America's concert revenues are down in 2010 following a stellar 2009.

Promoters will give a full refund to those who had bought tickets, adding that Rihanna "hopes to make it up to them on the next tour".