Prosecutors said May 20 they plan to call a number of witnesses, including two of Hunter Biden’s former girlfriends, in the upcoming trial against President Joe Biden’s son.

One of the ex-girlfriends will testify that while she was in a relationship with Mr. Biden from December 2017 to October 2018, she “observed the defendant using crack cocaine frequently—every 20 minutes except when he slept,” according to a filing from special counsel David Weiss’s team.

The other ex-girlfriend, who was in a relationship with Mr. Biden in October 2018, and at other times, saw Mr. Biden using drugs on multiple occasions, prosecutors said. She'll testify that when Mr. Biden stayed at her home in 2018, she and her children searched his belongings and vehicles in a bid to help him get sober and discovered drugs.

Prosecutors are also planning to call Kathleen Buhle, Mr. Biden’s ex-wife.

Ms. Buhle divorced Mr. Biden in 2017 but through 2018 she kept checking his vehicle sometimes to try to keep their children away from drugs, according to the filing. She found drugs or drug paraphernalia about a dozen times when searching the vehicle, prosecutors said. “I also found a few crack pipes. I took them out because our daughter was driving the car,” she wrote in one text message to Mr. Biden on March 9, 2018.

The witnesses will help establish that Mr. Biden was a drug addict when he entered a gun store on Oct. 23, 2018, and falsely checked a box that said he was not an unlawful user or, or addicted to, any drugs, prosecutors said. Checking the box enabled him to buy the gun, a speedloader, or an accessory that enables rapid reload, and 25 rounds of ammunition. Photographs and videos of Mr. Biden smoking crack are among the government’s intended exhibits.

Mr. Biden is headed to trial on charges that he violated federal law that prohibits making false statements while acquiring guns or ammunition, and another law that bars people who use or are addicted to drugs from possessing firearms. Mr. Biden faces up to 25 years in prison if convicted.

Portions of Mr. Biden’s memoir will also be introduced during the trial, including how Mr. Biden wrote in early 2018 that he was able to “find crack anytime, anywhere” and described himself as “someone who’s up twenty-four hours a day, smoking every fifteen minutes, seven days a week.”

Mr. Biden later in the year flew to Delaware, where he grew up, landing on the morning of Oct. 6, 2018, according to flight records obtained by the government.

Several weeks later, one of Mr. Biden’s ex-girlfriends took the gun from his car and discarded it in a dumpster in a grocery store. Law enforcement officials found it there and launched an investigation, which ultimately resulted in the federal charges.

That ex-girlfriend will “establish that the defendant possessed the gun and she discarded it in an outdoor trash receptacle at the Janssen’s Market in Wilmington, Delaware after removing it from his vehicle,” prosecutors said. Text messages sent by Mr. Biden will help establish that he possessed the gun, including one in which he wrote, “did you take that from me...? Are you insane.” The government also plans to present statements made by Mr. Biden to police in which he acknowledged buying and possessing the firearm.

From left to right: Hunter Biden, Kathleen Buhle, Joe Biden, and Jill Biden in Virginia in a file image. (Jim Bourg/Pool/Getty Images)

Other Witnesses

In their trial brief, prosecutors disclosed other people they want to call as they make their case to a jury.

FBI special agent Erika Jensen will introduce Mr. Biden’s text messages, prosecutors said. Joshua Romig, an agent with the U.S. Drug Enforcement Administration, will be called to opine on coded language used in Mr. Biden’s texts, including “Party Favor” and “10grams.”

Jason Brewer, a forensic chemist, will testify about his analysis of white powdery substance residue on a leather pouch that was with Mr. Biden’s gun and has been until recently in a Delaware State Police vault, according to the special counsel. FBI special agent

Multiple witnesses will testify about the response to the grocery store, including Delaware State Police senior corporal Joshua Marley and former Delaware State Police lieutenant Millard Greer. An employee at the gun store, Starquest Shooters, is also expected to talk about selling the gun to Mr. Biden and observing Mr. Biden fill out the federal form on which he said he was not a drug user or addict.

“The government anticipates including several other witnesses on its witness list and may call those witnesses in its case-in-chief or in rebuttal,” prosecutors said. They said the summary in their filing of evidence and witnesses was “not an exhaustive list.”

Mr. Biden has pleaded not guilty to all three counts. He requested the case be dismissed for alleged vindictive prosecution and other reasons, a request turned down by a federal judge. An appeal of that order was also rejected. Mr. Biden this week asked the full appeals court to reconsider that ruling.