The state is preparing for a nearly 500-bed expansion of the Middlesex House of Correction even as the project draws fire from Billerica’s Board of Selectmen.

State officials say the $37 million expansion is needed temporarily to relocate about 400 pre-sentence detainees from the overcrowded Middlesex Jail  in East Cambridge to the Billerica facility while the state seeks to develop a new combined courthouse and jail complex in southern Middlesex County.