The top aide to retired Pope Benedict XVI is insisting he resigned freely, amid conspiracy theories that Benedict's resignation was forced and the election of Pope Francis was thus invalid.

Monsignor Georg Gaenswein gave an interview to Italian daily Corriere della Sera on the second anniversary of Benedict's announcement that he would become the first pope in 600 years to step down.

Gaenswein said: "Benedict XVI is convinced that the decision that he took and communicated was right. He has no doubts."

Italian writer Antonio Socci wrote a book last year arguing that, for several technical and theological reasons, Benedict is still pope and Francis' election is invalid.

Last month, retired Kazakh Archbishop Jan Pawel Lenga wrote an open letter claiming, without evidence, that Benedict's resignation was forced.