Theguardian_ People’s love of Frida Kahlo’s vibrant art is matched by fascination with her colourful private life. Now the battle to win greater attention for her talent – above and beyond her extraordinary, painful personal story – faces another potential knock.

A documentary about the Mexican artist is to reveal a secret suspicion that endures within the family of her husband and great love, the renowned muralist Diego Rivera.

Early next month, a three-part BBC series, Becoming Frida Kahlo, will feature an interview with Rivera’s grandson, Juan Rafael Coronel Rivera, in which he states his belief that his grandfather “probably” ended Kahlo’s life in a last act of love. It is testimony that is intended to stop speculation about the artist’s “mysterious” final hours, but it will fan new flames among her millions of admirers.

Coronel Rivera, a Mexican art journalist and photographer descended from Rivera and his first wife, will suggest that Rivera “helped her”, and that he does not “feel like it’s something wrong”. In the final part of the BBC2 documentary, he argues: “If your companion of life says, ‘I’m tired, I really want to go now, help me’ – well, maybe you try.”

Kahlo, born in 1907, was plagued by ill health throughout her short life. A polio survivor, she was also severely injured in a road accident and had increasing difficulty moving. She also endured miscarriages, which she notoriously portrayed in a series of groundbreaking and powerful paintings.

Equally cruel, however, were her troubled emotional fortunes. Her passionate relationship with Rivera, whom she had met before he became controversial as a Communist campaigner for social change in the United States, was volatile from the first. His many affairs, including with her own sister, Cristina, and with the Hollywood star Paulette Goddard, continually jeopardised their marriage.

“I agree one 100% with Juan,” said the Kahlo expert Luis-Martin Lozano, speaking from Mexico this weekend. “Frida’s family always had this idea that she could have been alive for some years. It is sad to say, but suicide was always also a possibility. Kahlo wrote about it in her diary, but her love for Diego prevented her. She did not want to leave him. But the issue must have come up between them.”

Diego Rivera and Frida Kahlo at an exhibition of Jewish portraits by Lionel Reiss in New York. Photograph: Bettmann

The documentary, executive-produced by Nancy Bornat, looks at key creative moments in Kahlo’s career in the light of new evidence about her state of mind and physical health. In a series of interviews filmed in Mexico, where her famous Blue House is now a leading tourist attraction, the documentary depicts a complete picture of her work and her love life, which included affairs with Leon Trotsky, the exiled Russian Communist leader, and with Jacqueline Lamba, the young wife of the influential French surrealist artist Andre Bréton.

Nevertheless, it was Rivera who first spotted Kahlo’s artistic gifts, and he kept a secure grip on her affections until her death in 1954, at 47.

“Rivera was careful. He never told her how to paint, he had too much respect,” said Lozano, who appears in the series. “He was her first fan and told her to be herself. He realised she had potential and encouraged her to go on. He was a great provider for her, at the same time, and you cannot deny she would not have blossomed so well without him. Rivera provided stability but her paintings are hers.”

The final phase of Kahlo’s life, after she had been celebrated abroad with exhibitions in New York and Paris, began after a trip to San Francisco where Rivera had moved with Goddard after their divorce. Bereft in Mexico, Kahlo followed him and the pair, reunited, decided to remarry in 1940 on the understanding that while neither of them were likely to reform, they could not be apart.

Back in Mexico City they lived entwined, but separate, in neighbouring homes. The darkest months came with the amputation of Kahlo’s leg and her growing dependence on drugs and alcohol to deal with her discomfort and her doubts about her artistic worth.

At the time she wrote to Rivera: “They want to hurt my pride by cutting a leg off. When they told me it would be necessary to amputate, the news didn’t affect me the way everybody expected. No, I was already a maimed woman when I lost you, again, for the umpteenth time maybe, and still I survived … I am not afraid of pain and you know it.

“It is almost inherent to my being, although I confess that I suffered, and a great deal, when you cheated on me, every time you did it, not just with my sister but with so many other women.”

Lozano explains that Rivera avoided her during the day because he found her pain hard to witness: “We will never know if he ended her life, but if she asked him, I don’t see him saying ‘no’.

“Kahlo’s family have also talked about the possibility that he helped her die. Her niece said a little about this, although it may be taboo in Rivera’s family.”

Kahlo was given a state funeral at Mexico City’s Palace of Fine Arts. One of her famous self-portraits, featuring a small image of Rivera implanted on her own forehead, sold at auction recently for $35m.

“Kahlo must not be victimised, although there’s obviously a lot of pain in the iconography. You can admire the quality and the craftsmanship and her work at the same time. I don’t think she should be seen as a martyr,” concludes Lozano. “Her painting is not all about pain. It’s about a woman becoming an artist. She was a true pioneer.”