NEW YORK (CNNMoney.com) - The Justice Department on Wednesday again voiced fear over a potential shutdown of Research in Motion's BlackBerry service in the U.S., saying it remains unclear how a blackout would impact government workers.

"There are still a number of serious questions to be answered as to how an injunction can be implemented so as to continue BlackBerry service for governmental and other excepted groups," the department said in a brief filed with a federal district court in Virginia.

BlackBerry blackout could be costly A shutdown of the wireless email system would have a profound financial impact on businesses.(Full story.) More BlackBerry stories: • Goldman: BlackBerry shutdown not likely • BlackBerry held hostage • Setback for BlackBerry maker

U.S. patent holding company NTP has said an injunction would not affect BlackBerry products used by U.S. federal, state, or local governments, but the government wants more assurance that its users will be exempted from any blackout.

The threat of a BlackBerry shutdown stems from a legal battle RIM is locked in with NTP. The case goes back to 2001, when NTP filed a suit against RIM claiming that the BlackBerry infringed on its patents. A jury found in favor of NTP and awarded the company damages.

The next milestone in the U.S. legal case could come later this month, when a federal judge is set to hold a hearing Feb. 24 on whether to shut down the wireless email provider's operations in the U.S.

RIM has successfully won patent infringement cases in England and Germany, where courts have ruled in its favor against Luxembourg-based patent holding company InPro.

Also on Wednesday, the U.S. Patent Office issued a "non-final" rejection of an NTP patent involved in the dispute, sending RIM (down $1.23 to $72.38, Research) shares soaring nine percent in Nasdaq trading.

The patent office has now issued non-final rejections on all five of the patents at the center of the legal tussle between NTP and RIM.

The decision works in RIM's favor, but it shifts the weight in the battle ever so slightly, ICAP analyst Richard Williams said. "It takes the heavy weight off RIM's shoulders. The basis of NTP's infringement claims now has been disallowed," he said.

But, the judge who is hearing the case in U.S. district court has not been swayed by previous patent office rulings.

Furthermore, NTP has the right to appeal the decisions, which could extend the battle by another two years, Williams said. That's not ideal, especially since RIM's rivals are busily launching competing products.

James Wallace, lead trial counsel for NTP, said the latest ruling from the patent office doesn't affect NTP's ongoing patent infringement case against RIM.

"This is precisely what the patent office said it was going to do," he told CNNMoney.com.

Wallace said NTP would bring the matter to the patent appeals board if the patent office were to issue final rejections of NTP's patents.

Final decisions from the patent office aren't likely to be made until the end of February. Final rejections could deny the validity of the patents and could potentially discredit NTP's claims against RIM, Goldman Sachs analysts said in a report last month.

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