Pope Francis has moved Pope John Paul I, the pope who reigned for only 33 days in 1978, a step closer to possible sainthood just days after the Vatican endorsed a new book debunking decades of conspiracy theories about his sudden death.

The Vatican said on Thursday that Pope Francis signed a decree declaring that Pope John Paul I had lived a life of heroic virtue.

The Vatican must confirm a miracle attributed to his intercession for the late pope to be beatified, and a second miracle for him to be made a saint.

Earlier in the week, a book titled Pope Luciani: Chronicle Of A Death went on sale in Italy.

It stated that Pope John Paul I, the former Cardinal Albino Luciani, died of a heart attack at the age of 65.

The author, journalist Stefania Falasca, was involved in the beatification cause and had access to confidential Vatican documents, including  Pope John Paul's medical file.

The book features a preface by the Vatican secretary of state, Cardinal Pietro Parolin, who said it was time to move on and appreciate Pope John Paul I's legacy and historic value to the Catholic Church.

Conspiracy theories have swirled for decades that Cardinal Luciani was murdered in the Apostolic Palace as part of a plot involving the Vatican's scandal-marred bank.

His September 28 1978 death led to the election a few weeks later of the Polish cardinal, Karol Wojtyla, who took inspiration from his predecessor and named himself Pope John Paul II.