Dan Evans was being cross-examined by Mr Coulson's defence lawyer during a third day in the witness box at the Old Bailey.
Asked about who knew about the illegal activity, Evans raised his voice and declared: "The truth is that Andy Coulson knew exactly what was going on on his watch."
Mr Coulson looked up in the dock and exchanged stares with the witness, who has already admitted his part in hacking into the voicemails of celebrities.
Mr Evans, 38, was caught trying to hack into designer Kelly Hoppen's voicemails, and then lied to police, the court heard.
The court heard that Evans has already admitted conspiracy to hack phones.
He also pleaded guilty to conspiracy to commit misconduct in public office, and perverting the course of justice.
Mr Coulson, 46, from Charing, Kent, denies conspiring to hack phones and conspiring to commit misconduct in a public office.
Seven defendants deny all the charges against them.
The trial was adjourned.