The Vatican has admitted that it altered a photo sent to the media of a letter from the retired pope Benedict XVI which praised Pope Francis, blurring a paragraph in which Benedict said he would not read about Francis's theology.

In the part of the letter that is legible and was reported this week, including by Christian Today, Benedict says there is an 'inner continuity' between his pontificate and that of Pope Francis, and praised his successor as a 'man of profound philosophical and theological formation'.

He also dismisses the 'foolish prejudice' of critics who believe the conventional wisdom that Francis lacks theological formation and Benedict has 'little understanding of the concrete lives of today's Christians'.

However, it has now emerged that the Vatican blurred the two final lines of the page, where Benedict begins to explain that he did not read books about Francis's theology and cannot contribute a theological assessment of Francis as requested because he has other projects and not enough time.

The Vatican had released an 11-part series called 'The Theology of Pope Francis' earlier this week. Mgr Dario Edoardo Viganò, prefect of the Secretariat for Communication, sent an advance copy to Benedict, who responded with his letter earlier this year.

To mark the official launch of the series which coincided with the fifth anniversary of Francis's election, Viganò published the first two paragraphs of the letter.

But in the unpublished paragraph of the letter, according to a translation by Sandro Magister's blog reported by the Catholic Herald, Benedict XVI wrote: 'Nonetheless, I do not feel that I can write a brief and dense theological page about them because for my whole life it has always been clear that I would write and express myself only on books that I had also truly read. Unfortunately, even if only for physical reasons, I am not able to read the eleven little volumes in the near future, all the more so in that I am under other obligations to which I have already agreed.'