Pope Francis will write a major document on 'fake news', he announced to his nearly 40million Twitter followers on Friday.

The pontiff will address the topic on the Catholic 'World Communications Day' 2018 under the theme of 'the truth will set you free', quoting John 8.32.

The annual event falls on May 13 and Francis' intervention will be released on January 24, the feast of St Francis de Sales, patron of journalists.

'The Church too wishes to offer a contribution," to the discussion of fake news,' a statement from the Vatican said.

It added it was 'proposing a reflection on the causes, the logic and the consequences of disinformation in the media, and helping to promote professional journalism, which always seeks the truth, and therefore a journalism of peace that promotes understanding between people.'

Francis' message could set him on another collision course with US President Donald Trump who frequently brands major international media outlets 'fake news' if he dislikes their content.

The pair has clashed a number of times after Francis said Trump could not be a Christian if he was more interested in building walls than bridges.

'A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian. This is not the gospel,' he said.

Trump, who had once said that Francis 'seems like a pretty good guy', called the Pope's disparaging of his faith 'disgraceful'.

The Vatican said the theme 'relates to so-called "fake news", namely baseless information that contributes to generating and nurturing a strong polarisation of opinions. It involves an often misleading distortion of facts, with possible repercussions at the level of individual and collective behaviour.'