NEW YORK ( TheStreet) -- There may be more trouble on the way for Ambac Financial Group ( ABKFQ) after the U.S. Justice Department filed a lawsuit over the restructuring of the monoline insurer. The suit, filed on behalf of the Internal Revenue Service, seeks to block a state court's restructuring plan that could stop the IRS from collecting $700 million in taxes, according to a Reuters report. Ambac Financial Group received approval in January to restructure its insurance unit -- Ambac Assurance Corp. -- a decision that would allow monoline to write new business. The U.S. government attempted to dissolve the injunction in January and move the Ambac Assurance case to a federal court. --Written by Maria Woehr in New York. To contact the writer of this article, click here: Maria Woehr. To follow the writer on Twitter, go to http://twitter.com/newsgirlmw. To submit a news tip, send an email to: firstname.lastname@example.org.