DALLAS (AP) — A federal judge is siding with the Biden administration and blocking JetBlue Airways from buying Spirit Airlines, saying the $3.8 billion deal would reduce competition.

The Justice Department sued to block the merger, saying it would drive up fares by eliminating Spirit, the nation’s biggest low-cost airline.

JetBlue argued that the deal would help consumers by making JetBlue a stronger competitor against bigger rivals that dominate the U.S. air-travel market.

U.S. District Judge William Young, who presided over a non-jury trial last year, said in the ruling Tuesday that the government had proven “that the merger would substantially lessen competition in a relevant market.”

Shares of Spirit Airlines Inc. plunged more than 53% almost immediately.