The woman who captured Robert Capa's heart

He was the legendary war photographer, a man who alternated dodging death on battlefields with a glamorous, star-studded life-style; she was a self-effacing, left-wing intellectual who preferred to stay out of the limelight, hidden behind the camera's lens.

For decades, their relationship has been a well-kept secret: but now the story of Robert Capa, whose images of the Spanish Civil and Vietnam wars made him one of the greatest photographers of all time and his teenage sweetheart, Kati Horna, can be told.

Both Capa, who was born Endre Friedmann in 1913, and Horna, who was born Kati Deutsch in 1912, grew up in Budapest. They met as teenagers, and quickly became inseparable, according to Horna's daughter, Norah. She says that, though Capa's relationships with women including Hollywood actress Ingrid Bergman and photographer Gerda Taro have become the stuff of legend, it was to Kati that he gave his heart. As youngsters, both were members of the same left-wing intellectual movement, and both took up photography with gusto.

They sometimes used one another as subjects: Horna's first portrait was of Capa, and he took one of her – photographs her daughter cherishes to this this day. "When Capa met my mother he was instantly struck by her," says Norah Horna. "She was a banker's daughter from the prosperous area of Buda; he was from humbler origins, from the industrial area of Pest. But he was fascinated by her, and he remained fascinated by her throughout his life. They spent a great deal of time together, and their relationship affected each one very deeply."

The two were separated when, at the age of 18, Horna went to live in Berlin. But their paths crossed again a few years later in Paris, where the now-renamed Capa (he reinvented himself in order to be "Americanised") had his studio. From there, he made many trips to Spain to photograph the Spanish Civil War. Horna followed him. But while Capa received commissions with the prestigious Life magazine, Horna preferred to work for lesser-known titles such as Umbral, which supported the anarchist cause she believed in.

While Horna focused on the effect of war on women, children and other non-combatants, Capa was at the front, taking the photographs which would make him one of the earliest celebrity photographers. His best-known image was the Falling Soldier, a man photographed in Spain in 1936 on the point of death, at the exact second he was hit by a bullet. But its authenticity was later called into doubt, and it remains one of the most controversial photographs ever taken.

According to Norah Horna, who lives in Mexico City to which her mother fled at the outbreak of the Second World War, her mother provided Capa with something entirely different from his other girlfriends. More than any of them, says Norah, her mother understood him: she understood where he had come from, and she understood the inherent insecurities that came from his origins, which were humbler than hers.

Now Horna's work is to be shown in the UK for the first time, at an exhibition that opens next weekend in Chichester. "The majority of her war photographs are compassionately observed scenes from behind the front lines," says Dawn Ades, professor of art history at Essex University. "She also created some striking images using superimposition. In one picture, The Aragon Front, two negatives are superimposed and then printed, so that an old woman and child appear as though ghosts in the middle of a house ruined in the fighting. The image is specific to the Spanish conflict, but it could express the fate of victims in any war."

When photographing civilians during a conflict was almost unheard of, Horna pioneered a new approach. From Barcelona, Aragon and Catalonia, she sent frames which documented the pain, confusion and suffering borne by the ordinary people of Spain, regardless of their political affiliation.

Until Horna died in 2000, Norah – her only child – had no idea how extensive her mother's archive of pictures from Spain was. "It had all been hidden away," she says. "She spoke of her experiences in Spain very rarely."

The UK exhibition has come about because of Horna's connection with the English surrealist painter Leonora Carrington, whom she befriended in Mexico. The two women – along with a Spanish painter called Remedios Varo – formed a close-knit artistic group, surrounded by other European émigrés, including Carrington's husband Imre Weisz (known as "Chiki"), another boyhood friend of both Horna's and Capa's in Hungary.

Weisz, who was Capa's business partner during the Spanish Civil War, was responsible for helping to save a cache of Capa negatives known as the Mexican suitcase which came to prominence in 2007 after they had been missing for many years. Capa had left all his negatives in Paris when he fled Paris for New York in 1939 as German forces approached France. In a letter written on July 5 1975, Weisz wrote that, as the Nazis approached, he "put all Bob's negatives in a rucksack and bicycled it to Bordeaux to try to get it on to a ship to Mexico". Many years later, the suitcase was discovered in a house in Mexico City. Its contents have since become part of the collection at the International Center of Photography in New York.

Before she left Europe for Mexico, Horna met the man who become her life partner – Jose Horna, a sculptor and craftsman. According to their daughter, Jose was a better match for her mother than Capa, and even when, in New York on the way to a new life together in Mexico, Capa begged her to stay with him, Horna refused.

The following year Capa visited Mexico. While he was there he told Horna he was fed up with the demands of his difficult and dangerous work on battle front lines. All he really wanted to do, he said, was to settle down with her and have 12 babies. "But my mother only smiled, because she wasn't available. She was with my father by that stage," says Norah.

All the same, she says, when her mother heard, 14 years later, that Capa had been blown up by a landmine in Vietnam, she was inconsolable.

Could Horna, who went on to become one of Mexico's most respected documentary photographers, as well as a significant surrealist photographer, hold the key to one of the most enduring mysteries of photography – the whereabouts of the negative of the Falling Soldier? Experts believe the negative, if found, would settle the question once and for all of whether the soldier's "death" was staged.

Norah Horna will not be drawn. "The important thing now," she says, "is that my mother's work, which has been buried for so long, is about to be more widely known. She was an extraordinary photographer with a talent at least equal to many of those of her era who found fame."

Surreal Friends – an exhibition of the work of Kati Horna, Leonora Carrington and Remedios Varo opens at Pallant House Gallery in Chichester on Saturday June 19. www.surrealfriends.com