10:20 AM PT -- Hickerson appeared in court this morning and pled not guilty to the charge. His bail was set at $50k. The judge also issued a protective order that prohibits Hickerson from being 100 yards near Panettiere or contacting her. He'll be back in court next month.

Hayden Panettiere's boyfriend is in serious trouble following his arrest for domestic violence ... 'cause TMZ's learned he's now facing a charge that could land him in prison.

The L.A. County District Attorney's Office has charged Hayden's BF, Brian Hickerson, with 1 count of felony domestic violence. If he's convicted, Brian faces up to 4 years in prison.

TMZ broke the story ... Brian was arrested earlier this month after an argument with Hayden allegedly turned physical. As we told you ... the couple had been out drinking before an argument ensued at their pad. Law enforcement sources initially said the injuries were marks and redness, but we're now told the injuries were far more serious.

Cops arrested Brian almost immediately after the altercation after determining he started the fight. Hayden and Brian have been dating since last summer.