Every day 19-year-old Lorenzo Delloye makes a call to a live radio programme. He chats about what he has been doing, about his friends, his hobbies, his university studies. He tells his mother that he loves her. That he misses her.

Once a year, he says, he wishes her a happy birthday, “even if ‘happy’ is ironic because obviously it is not a happy birthday”. His mother, Ingrid Betancourt, 46, has been held hostage for six years by leftist guerrillas in a remote Colombian jungle. Abattered old radio, which does not always work, is her only link with the outside world – and to her son Lorenzo.

“I tell her about my life because it is like fuel for her, to keep her going,” he tells me. His dark eyes reflect a deep melancholy: he is convinced that unless she is released soon, his mother will die. “Either we get my mother out of there or we’ll be making a grave for her,” he says. “It is a matter of weeks before her life fades away.”

Lorenzo was 13 the last time he saw his mother, who was a presidential candidate in Colombia when she disappeared in 2002 on an ill-advised visit to a rebel-held part of the countryside. Since then, Lorenzo says, his love and admiration for her have grown over the years. So has his scorn for her captors.

“I hate them with all my heart,” he says. “They took her from me as a teenager, just when I needed her most.”

Betancourt has long had close links to France. Last week Nicolas Sarkozy, the French president, dispatched a mediator and a doctor to Colombia, warning that if she died in captivity it would be a “humanitarian tragedy, a crime” for which France would hold Manuel Marulanda, the rebel leader, personally responsible.

Details of her decline from a combination of ailments, including hepatitis B, have been provided by other hostages released recently – and by one who escaped after a 17-day march through the jungle. But nothing summed up her plight as poignantly as the 12-page letter she wrote in December to her mother and her children, in which she said that she thought sometimes death would be a “sweet option”, allowing Lorenzo and her daughter Melanie, a student in New York, to get on with their lives.

This extraordinary letter, from which we publish an extract below, was handed to the family as part of a “proof of life” package that included film of Betancourt looking desperately thin and ashen-faced as she sat with her head bowed in a camp somewhere in the Colombian Amazon. The children were torn between horror at how ill their mother looked and joy at the first evidence in years that she was still alive.

“We know that you are at the end of your tether,” they have written in a reply that is being published in Britain next month, along with Betancourt’s letter.

“We can imagine how difficult it is to dredge up one last reserve of strength, to tackle, again and again, one more night of suffering, one more forced march into hell, one more humiliation. We are going to get you out of there.

“In these terrible moments of doubt and defeat, say to yourself, I beg of you, that just a little farther on, beyond the jungle, we are here, we are thinking of you . . . Your strength has always carried us. It is our turn now to carry you, to take care of you.”

In her own letter, Betancourt addresses Lorenzo affectionately as “lollipop”, “my angel of light”, “my wonder child, my tiny scrap of the sun”.

“I love you so much, my darling,” she writes, revealing that she has managed to hear his voice – now so much deeper than she remembered – twice on the radio in the past year.

Herself a graduate of the prestigious Institute of Political Studies in Paris, she tells him how proud she is to hear that he is studying law and economics at the Sorbonne, and then urges him to take up political science and do a PhD. Her maternal admonitions have brought back memories for Lorenzo, who is torn between pursuing a career as a musician and the desire to satisfy his mother’s demand that he continue his studies.

“I could hear her voice speaking in that letter,” he says. “She was always telling me what to do; giving me a hard time for not studying hard enough, for not practising the guitar. She would always win those fights.”

As we sit in a cafe down the road from the Sorbonne, he tells me that he finds it “surreal” imagining his mother in what she describes as her “green prison”.

“Here I am, having this hot chocolate in this cafe and there she is in the jungle, in pain, suffering. I am free. She is not.”

His mother has been punished each time she has been caught trying to escape, he adds. After each attempt, “they take away her radio, so that she cannot hear us. It is a cruel horror”.

Her life is now thought to be in grave danger if she does not receive a blood transfusion soon. According to French officials, the French air force plane sent to Colombia last week is fully equipped to provide emergency medical attention.

No one knows, though, whether the guerrillas will even agree to meet the French delegation, let alone allow it to provide treatment for Betancourt. An attempt by the previous French government to negotiate her release ended in acute embarrassment when diplomats and intelligence operatives from Paris were arrested in the Amazon on suspicion of being drug traffickers.

Mediation by Hugo Chavez, the maverick president of Vene-zuela, helped to win the release earlier this year of Clara Rojas, Betancourt’s running mate, who had given birth to a child while in captivity. The father was one of the guerrillas. Further negotiations were frozen, however, after an assault by the Colombian army on a guerrilla camp in Ecua-dor, in which the deputy commander of the rebels was killed.

The focus of regular vigils and marches in Paris, Betancourt lived in France as a child with her father, a delegate at Unesco, and her mother, a former Miss Colombia and congresswoman.

She married Fabrice Delloye, a French diplomat whom she had known since university, but the couple separated when she returned to Colombia in 1989 after the assassination of Luis Carlos Galan, a presidential candidate who had been battling corruption. Betancourt decided she wanted to join the struggle for cleaner government and ran for a seat in parliament in 1994.

After receiving death threats – an occupational hazard for Colombian politicians – she sent Lorenzo and Melanie to live with Delloye for their own protection. She later remarried, but relations with her former husband remained “incredibly good”, according to Lorenzo. Until the kidnapping, he says, they were “one big happy family”. Indeed, Delloye is at the forefront of the campaign to free Betancourt.

On the day she was snatched, Lorenzo was at a friend’s flat when he got a call from his father, telling him to go outside. Delloye was waiting for him with Melanie in the car.

“When they told me what had happened, I thought it would be like in the films,” says Lorenzo. “I thought that they would ask for a ransom, that we’d pay it and that she’d be quickly released. How wrong can you get?”

The Revolutionary Armed Forces of Colombia, or Farc, as it is known by its Spanish acronym, is one of an alphabet soup of rebel organisations that has been battling the government in Bogota for decades. It funds itself from drug-trafficking and kidnap.

The rebels have offered to release Betancourt and other “political” prisoners in exchange for the release of rebels held by the government. Complicating negotiations, however, is a demand that the Colombian army withdraw from a large swathe of territory. The government has flatly refused.

Today Lorenzo will attend what he calls “a last march” for his mother, when thousands are expected to gather in Paris to demand her immediate release. He hopes that she will hear about it on her ancient radio.

“Let our words, which come to you drip by drip over the radio, be your energy,” Lorenzo and Melanie tell her in the letter that she has probably never received. “We will not forsake you, Mama.”

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