The ancient order of the Knights of Malta has told Pope Francis that his decision to launch an investigation into the dismissal of a top official over a condom scandal is "unacceptable" in an extraordinary rebuke of the pontiff.

The group, formally titled the Sovereign Order of Malta, said the replacement of its grand chancellor was an "act of internal governmental administration of the Sovereign Order of Malta and consequently falls solely within its competence."

Pope Francis on Thursday appointed a five-member commission to investigate the December 8 sacking of Albrecht von Boeselager amid evidence that Francis' own envoy to the group, conservative Cardinal Raymond Burke, helped engineer it without his blessing.

One charge used against von Boeselager concerned a program that the order's Malteser International aid group had participated in several years ago with other aid groups to help sex slaves in Myanmar, including giving them condoms to protect them from HIV infection.

Church teaching bars the use of artificial contraception.