Bill Cosby’s lawyers will ask a judge to throw out the only criminal case lodged against the TV star from the dozens of accusations that he molested women.

The defence will argue that Cosby had a deal with a suburban Philadelphia prosecutor in 2005 that he wouldn’t be prosecuted and should testify freely in accuser Andrea Constand’s civil lawsuit.

That testimony, released only last year, prompted the successors of former Montgomery County District Attorney Bruce L. Castor Jr to reopen the case and ultimately charge the 78-year-old Cosby with felony sexual assault. Cosby has not yet entered a plea.

• Cosby charges won’t proceed
• Cosby’s wife delays testimony
• Bill Cosby charged with sexual assault

Cosby admitted in the deposition that he had a series of affairs with young models and actresses; had obtained quaaludes to give women before sex; and gave accuser Andrea Constand three pills before a January 2004 encounter at his home. He called it consensual but she said she was drugged and violated.

In an unusual twist, Castor is slated to be the defence’s key witness on Tuesday. Castor insists that he forged an oral “non-prosecution” agreement in 2005 with Walter M. Phillips Jr, a Cosby lawyer who died last year.

It’s not clear if anyone can corroborate Castor’s account. However, Temple University board chairman Patrick O’Connor represented Cosby at the deposition. He still represents Cosby in some civil matters.

Anne Poulin, a law professor at Villanova University, said those involved in the case probably would want to hear from O’Connor “on the theory that, certainly as the lawyer representing Cosby in the civil case, he therefore needed to know what (Cosby’s) exposure was to criminal prosecution before he was deposed”.

Poulin believes the defence has a high bar to meet to get the case thrown out early on. At the same time, she said, “if they can win without this ever going to trial, then they’ve done their client a big service”.

Kevin Steele, the new county district attorney, believes Cosby needed an immunity agreement – in writing – to avoid prosecution. He has said he has no evidence that one exists.

“I never made a deal that involved criminal cases that was not in writing,” Lynne Abraham, a long-time judge and district attorney in Philadelphia, said on Monday.

“This is, to me, is an extremely unusual arrangement because the prosecutor, Bruce, has made a deal not involving a criminal case, but a civil matter.”

The lawyer who served as Castor’s top assistant in 2005 could also be called to testify.