The Blackberry has become an executive status symbol

Research in Motion agreed in March to pay $450m (�247m) to settle a 3-year legal battle with patent-holder NTP, which alleged infringements by RIM.

RIM has now asked a US court to enforce the settlement, saying it has hit an "impasse in the process".

RIM boss James Balsillie said NTP was refusing to honour its obligations.

"RIM is not looking to modify any terms", Mr Balsillie said in telephone conference with IT analysts.

Instead, it seems RIM wants to ensure the settlement payment goes ahead.

The Nasdaq-listed firm has requested the US District Court in Richmond, Virginia that heard the original case in 2002 to reopen it.

RIM agreed the settlement after it lost its appeal against the case. In 2003, the court imposed a ruling banning sales of Blackberries in the US market - its biggest - but implementation was frozen.

"We all thought this was over with and now it's taking centre stage again," said ThinkEquity analyst Pablo Perez-Fernandez. "A cloud of uncertainty has returned."