(Reuters) -Budget airline Spirit Airlines on Tuesday urged the Transportation Department to reject a proposed JetBlue Airways ( JBLU ) and United Airlines partnership, saying it was anticompetitive and will prompt large carriers to pursue similar deals.

United and JetBlue ( JBLU ) said in May the "Blue Sky" tie-up would allow travelers to book flights on both carriers' websites, while interchangeably earning and using points in their frequent flyer programs. Spirit said the deal would mean the smaller JetBlue ( JBLU ) "will become a de facto vassal of United." Spirit added "this anti-competitive tie-up involving a dominant legacy carrier will neutralize the competitive benefit of an existing low-fare competitor."