Patent holding company NTP is suing several major US wireless companies for infringing patents which it holds on email services.

The company claims that AT&T, Verizon Communications and Sprint Nextel are in breach of eight patents.

NTP is seeking royalty payments based on sales of phones, PDAs and other mobile devices.

NTP won a $612.5m out-of-court settlement in March 2006 from BlackBerry maker Research in Motion (RIM) following a four-year legal battle.

The judge in that case had been expected to rule in NTP's favour and order RIM to shut down its wireless email service in North America.

NTP's latest legal challenge was filed on 7 September in the same federal court in Richmond, Virginia where the RIM case was held.

NTP also filed a lawsuit against Palm in November 2006, alleging that the Treo Smartphone infringed on its patents.