PHILADELPHIA — Prosecutors will be allowed to use Bill Cosby’s decade-old testimony about his sexual past and his use of drugs in previous encounters with women at his sexual-assault trial next year, a Montgomery County judge ruled Monday.

The defense had claimed that the 79-year-old entertainer only agreed to testify in the 2005 lawsuit brought by accuser Andrea Constand because he had been promised by a prior prosecutor that he would never be prosecuted.

But in a ruling Monday, Judge Steven T. O’Neill concluded that Cosby no such promise ever existed.

“This court concludes that there was neither an agreement nor a promise not to prosecute, only an exercise of prosecutorial discretion,” he wrote.