The US Department of Justice (DoJ) has asked a judge to ensure that government users can continue to send each other email on their "essential" BlackBerry devices, even though the court has ruled against the maker of the products in a patent infringement case.

The US District Court for the Eastern District of Virginia may soon reinstate an injunction requiring Research in Motion (RIM) to stop selling BlackBerry devices and software in the US, but the DoJ, in a statement of interest filed with the court, argues that government workers would lose an important tool if they are no longer available.

"The United States has a considerable interest in this action, to assure that its use of BlackBerry devices and software... is not impeded and that the public interest is not substantially harmed by any injunctive relief," wrote Paul McNulty, US attorney for the Eastern District of Virginia, in the 8 November document. BlackBerry devices are widely used by government and congressional workers in Washington DC.

RIM does not maintain a database of governmental BlackBerry users, and it would be difficult for the company to identify such users, the DoJ document said. RIM identifies users by personal-identifier numbers, not by email addresses ending in government identifiers such as .gov or .mil.

Instead, it may be necessary to create a huge government 'white list' of users who should not have their BlackBerry service turned off if the injunction is reinstated, the DoJ said.

It also noted that the five patents related to wireless communications held by Virginia patent firm NTP are under review by the US Patent and Trademark Office. An NTP lawyer wasn't immediately available for comment.

In December 2004, the US Court of Appeals for the Federal Circuit upheld a 2002 jury decision awarding NTP $23.7m (about £14m) in damages for patent infringement. The appeals court sent the case back to the Virginia court for further deliberations. After the jury award, the Virginia court slapped an injunction on sales of BlackBerry devices and server software in the US, but the appeals court had vacated the injunction.

In March, two companies announced they had agreed to settle the dispute by having RIM make a $450m (£260m) payment to NTP in exchange for a perpetual licence to NTP's patents. However, the deal fell through; RIM thought the press release constituted a final agreement while NTP insisted the companies had never reached a definitive pact. RIM has asked an appeals court to enforce the terms of the settlement, and is expected to argue against an injunction by pointing out that it agreed to settle the case months ago.