Blackberry devices are used by three million people in the US

Officials at the US Patent and Trademark Office issued a final rejection of one of the five disputed patents owned by NTP.

It comes ahead of Friday's core court case when a US judge will decide if RIM has to shut down its service in the US.

NTP won such an order in 2003, but it was not enforced so RIM could appeal.

The crunch final decision - which could force RIM to halt Blackberry sales in the US - is now in the hands of the presiding judge in the case, US District Court Judge James Spencer.

The Patent Office's move is the first final decision on any of the disputed patents - although it has previously rejected all five on a "non-final" basis.

Long-running battle

Switching off the Blackberry service in the US would affect more than three million users.

Canada-based RIM said recently that it was negotiating with NTP, but described the US firm's offer to license the disputed technology as "untenable".

The tussle between the two companies dates back to 2002, when patent-holding company NTP successfully sued RIM in a lower court.

Earlier this month the US government asked a federal judge to hold off from a possible shutdown of the Blackberry service because of the system's popularity among key government employees.