Like many of his colleagues, Karl Rove is a "Crackberry" addict

Canadian firm Research in Motion (RIM) is embroiled in a patent row with a US firm, NTP, which is demanding the portable e-mail service be turned off.

But the system's popularity among government employees means the Justice Department wants guarantees that government users can be made exempt.

NTP won its first case in 2002, but the US patent office has since backed RIM.

On Wednesday, it made an initial judgement that a fifth NTP patent should be rejected.

It has already rejected the four other patents at the centre of the row.

But the next court hearing comes up on 24 February - months ahead of any likely final decision from the patent authorities.

The hearing could decide whether NTP's 2003 injunction on US sales and services of Blackberry - delayed by RIM's appeal - is reinstated.

The two parties reached a tentative $450m settlement on the dispute in March last year, but the deal fell apart in June and a court later ruled that the agreement was not valid.

'White list'

The Justice Department intervention follows the use of the government argument by RIM's lawyers.

They have claimed an "exceptional public interest" in keeping the service going, stressing that national security-related agencies are among RIM's biggest customers.

NTP has countered that a "white list" of government users could be given continued service while corporate users were shut out.

But the Justice Department told the judge that it had yet to see evidence that such a system could avoid causing "substantial hardship" to government functions.

Among the most avid users of the Blackberry - or "Crackberry", as some refer to near-addicted users - are White House staffers.