PORTLAND, Maine — The business partner of a Zumba instructor accused of using her dance studio as a front for prostitution is headed for trial next week after last-minute plea negotiations failed and a judge refused to let his lawyer withdraw from the case.

The case against Mark Strong Sr. expanded into a ‘‘behemoth’’ with a half-dozen agencies investigating, defense lawyer Dan Lilley told the judge, and Strong didn’t have the resources to pay for expert witnesses and forensic examinations.