United said the US federal Air Transportation Stabilisation Board on Wednesday rejected a request for a $US1.8 billion ($A3.21 billion) federal loan guarantee, setting off a run on the airline's stock and a credit downgrade in anticipation of default. United chief executive Glenn Tilton said a Chapter 11 bankruptcy, which allows companies to continue operating while seeking new arrangements with their creditors, was a likely option for United. Paul Whiteford, chairman of the United Airlines Pilots Master Executive Council, said Mr Tilton's remarks were "premature".