Two cardinals have criticised Pope Francis's approach to the clerical paedophilia crisis, saying homosexuality is to blame, not "abuse of power".

"Sexual abuse is blamed on clericalism. But the first and primary fault of the clergy does not rest in the abuse of power but in having gone away from the truth of the Gospel," US Cardinal Raymond Leo Burke and German Cardinal Walter Brandmueller said in an open letter to the heads of the bishops' conferences on Wednesday 20 February.

The letter comes a day before Pope Francis opens a historical summit at the Vatican on the protection of minors in a bid to tackle clerical abuse, an issue that has undermined the church’s credibility.

Read more 'Disappointed' abuse survivors demand to see Pope Francis at Vatican summit

The two cardinals have blamed a corruption of the "absolute moral law", referring to homosexuality, which they described as the "denial, by words and by acts, of the divine and natural law". They have called on the head bishops "to raise your voice to safeguard and proclaim the integrity of the doctrine of the church".

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Burke and Brandmueller are outspoken critics of Pope Francis and belong to the church's conservative wing. They say they were "among those who in 2016 presented to the Holy Father certain questions... (that) have not only not had any response, but are part of a more general crisis of the faith".

"The plague of the homosexual agenda has been spread within the church, promoted by organised networks and protected by a climate of complicity and a conspiracy of silence," they wrote. "We turn to you with deep distress! The Catholic world is adrift."

Catholic leaders will gather at the Vatican from Thursday until Sunday for the summit on child sexual abuse.