Authorities are investigating whether a letter circulating online, apparently written by disgraced Cardinal George Pell to supporters breaks prison rules.

The letter, dated 1 August, was posted on Twitter by an account called “Cardinal George Pell Supporters” on Friday night.

A Victorian Department of Justice spokeswoman on Saturday said prisoners are not allowed to post on social media or use the internet. They are also not allowed to ask others to post on their behalf.

“[The department] will thoroughly investigate this social media activity,” the spokeswoman said. “Any prisoner found to be contravening prison regulations faces disciplinary action.”

The spokeswoman would not confirm if the letter was written by Pell.

Pell was ordered to serve at least three years and eight months after being convicted in December of one charge of sexual penetration of a child and four charges of committing an indecent act with or in the presence of a child.

Pell was found to have raped one 13-year-old choirboy and sexually molested his friend in the sacristy of St Patrick’s Cathedral in Melbourne in 1996 when he was newly installed as archbishop. He molested the first boy again about a month later.

Pell is awaiting the ruling of the court of appeal, following a hearing on 5-6 June.