VATICAN CITY (Reuters) - A top Vatican official on Sunday issued a scathing open letter accusing an archbishop who launched an unprecedented attack on Pope Francis of mounting a “political frame job devoid of real foundation”.

Cardinal Marc Ouellet said in the detailed, three-page letter that calls by Archbishop Carlo Maria Vigano for the pope to resign because he had allegedly covered up sexual misconduct by a senior American churchman were “calumny and defamation”.