On the day that Pope Francis’ sex abuse summit is due to start, a potentially explosive book will be published claiming to lift the lid on gay priests in the Vatican and the double lives of senior officials.

The book, “In the Closet of the Vatican”, written by French sociologist and journalist Frederic Martel, reports that around 80 per cent of clerics working in the Roman Curia are gay – although not necessarily sexually active – and details how they adhere to an unspoken code of the “closet”.

After four years of gathering material which took him across the world Martel, a non-believer who is openly gay, spent around a week a month in Rome, sometimes staying in residences inside the Vatican or on Holy See property. He claims to have completed 1,500 interviews with 41 cardinals, 52 bishops and monsignors, 45 papal ambassadors or diplomatic officials, 11 Swiss guards and more than 200 priests and seminarians. The book is due to be published on 21 February simultaneously in 8 languages across 20 countries and will hit stores as bishops from across the world gather to discuss how to respond to clerical sexual abuse.

Defenders of “In the Closet” say that Martel will reveal the problems of a dysfunctional clerical culture that is in denial about sex, while others argue the timing of the book’s publication will once again unfairly conflate homosexuality with sexual abuse of children and intensify a witch hunt against gay priests.

Sources say that Martel’s research reveals that while some gay priests accept their sexual orientation and a number maintain discreet long term relationships, others live more extreme double lives through casual encounters and the use of male prostitutes, while others are in denial about their sexuality. He is said to argue that the intra-church battles of recent decades should be read through a closeted gay paradigm. Those with knowledge of “In the Closet” say the French writer reserves his harshest criticism for senior figures in the Church who have attacked homosexuality yet are secretly gay. One of the “rules” of the Vatican’s closet, Martel argues, is that the more a cardinal or bishop denounces homosexual behaviour or same-sex couples the more likely they are to be gay.

While, sources say, Martel does not focus on the sexual abuse of children, he alleges that the secretive sexual culture among clerics made it difficult for them to denounce priests accused of abuse. “In the Closet” claims that Pope Francis has sought to break up this pattern of behaviour by repeatedly condemning priests living a “double life”. At the same time, Martel argues that in doing so he has made the Church an unstable structure for closeted gay clergy, which in turn purportedly explains some of the opposition that Francis is facing from inside the Church.

One of the most explosive claims in the book, sources reveal, centres around deceased Colombian Cardinal Alfonso López Trujillo. Cardinal López Trujillo, a former President of the Pontifical Council for the Family, and said to have been for many years the chief obstacle to the canonisation of St Oscar Romero, is presented as both an arch-defender of the Church’s teaching on contraception and homosexuality while also using male prostitutes.

“It is not always easy to tell when Martel is trafficking in fact, rumour, eyewitness accounts or hearsay,” says a source with knowledge of “In the Closet.”

Among those Martel interviewed was German Cardinal Walter Kasper, who agrees that some in the Vatican hide their sexuality but adds that what most worries him is not sexual orientation, but whether the Church is helping people find the way of God.