The Vatican has begun an inquiry into a mysterious letter containing a historically-inspired death threat against Pope Benedict XVI's closest aide, an Italian news weekly reported.

The letter to Secretary of State Tarcisio Bertone -- a powerful but contested figure in the Vatican -- evokes a famous dispute between Giovanni Bosco, the founder of the Salesian order, and the Italian royal family, Panorama reported.

"Great funerals at court!" reads one passage of the letter -- the same phrase used by Bosco in a dark warning to then-king Victor Emmanuel II in 1854 against a disputed law that would have suppressed religious orders, the report said.

Several members of the royal family died shortly after Bosco's warning.

Contacted by AFP, the Vatican declined any comment on the Panorama report.

The letter accused Bertone of "not knowing how to take decisions and of choosing his aides on the basis of personal friendships," Panorama said.

In particular the letter reportedly criticised Bertone's decision -- due to be announced in coming days -- of replacing Carlo Maria Vigano, General Secretary of the Governorate, the body that manages the Vatican State.

Vigano is expected to be named as the new Vatican ambassador to Washington.

Bertone was named by Benedict as secretary of state -- effectively the Vatican number two -- in 2006. He worked for years before that with then cardinal Joseph Ratzinger at the Congregation for the Doctrine of the Faith.