The Duchess of Medina Sidonia was better known throughout Spain as the “Red Duchess”: a title bestowed upon her by the people - in addition to an already impressive list of noble ranks - for her determined and vocal opposition to the military dictatorship of General Franco.

She was descended from the 7th Duke of Medina Sidonia, commander of the ill-fated Armada, and was the 21st holder of Spain's oldest title, the Duchy of Medina Sidonia, established in 1445. Tracing her origins from King Ferdinand the Catholic and the Marquis of Gibraltar who recaptured the rock from the Moors in the 15th century, her great-grandfather was Antonio Maura, Prime Minister under King Alfonso XIII.

But the Duchess kicked against centuries of convention, enduring imprisonment and exile and eventually losing custody of her children. She always rejected the “Red” tag, arguing that communism was just as bad as Francoism when it came to loss of freedom.

She attracted international attention with her support for peasants demanding compensation after the Americans dropped four unexploded hydrogen bombs accidentally near Palomares, Almería.

Doña Luisa Isabel Álvarez de Toledo y Maura was born in 1936 at Estoril, Portugal, to where her family had fled at the outbreak of the Spanish Civil War. She was raised by her grandmother after her mother died. Her family had a liberal and mildly eccentric reputation; an ancestor relinquished his feudal privileges in the 19th century.

After the end of the civil war the family returned to their estates at Sanlúcar de Barrameda, near Cádiz. She later recalled the gulf between rich and poor: “I saw how my father and mother helped the poor with food, medicine and other things. My parents let me play with children from the village and I learnt that they were people too.”

She was educated at various convents from which she was regularly expelled. But she was fascinated by her family's history and spent many hours poring through its rich archives, cataloguing thousands of documents and manuscripts.

She inherited the title of Duchess of Medina-Sidonia on her father's death in 1955, and in the same year married José Leoncio González de Gregorio. They took up residence in the family palace, of Arab construction, in Sanlúcar, where she gave birth to and began raising her three children before embarking on a private land redistribution initiative by donating parcels of her vast landholdings to form rural co-operatives.

If this was regarded as little more than eccentric behaviour by the authorities, in 1964 she stepped across an invisible line between the two Spains by leading an illegal demonstration of fishermen striking for better wages. She was convicted but refused to pay a fine.

In 1966 a United States Air Force plane accidentally dropped four hydrogen bombs near the southern Spanish village of Palomares. Although they did not detonate there was radioactive leakage.

Local farmers were left uncompensated for their destroyed livelihoods. The Red Duchess highlighted the affair by leading a peaceful protest march, for which she was given a one-year prison sentence.

The Government soon rued the day it put her behind bars, because her noble lineage and diminutive five-foot figure became an emblem of the controversy. She was offered a pardon in return for repentance but she refused, eventually serving eight months in prison.

She and her husband had separated in 1960, and after her release from jail in 1968 she lost a bitter battle for custody of her children when the courts found her political activities evidence of instability.

A year earlier a French publisher brought out a Spanish language edition of La Huelga (The Strike), a gritty novel she had written about police brutality and judicial collusion in the violent suppression of a vineyard workers' strike. The book was banned in Spain.

A press tribunal, set up to handle offences against the country's censorship laws, cleared her of all charges, but in 1970 the verdict

was overturned by the Spanish Supreme Court, and she was sentenced to a month and a day in jail.

Instead of serving her time she left Spain for France where she wrote for Le Monde and Libération. She returned home in 1976 after Franco's death. She wrote other books with a social conscience; one about the exploitation of the peasantry by landlords and another about a US military base in Spain.

She left no one in any doubt of her aristocratic origins, being capable of an intimidating hauteur. Once she was before a court and addressed the judge as “you”. The judge reprimanded her and told her to address him as “Your Honour”, to which she replied: “In that case you will address me as ‘Excellency', since I am a duchess and a grandee.”

The Duchess was also the 18th Marquess de los Véles and the 18th Marquess de Villafranca de Bierzo, giving her the right to remain with her head covered in the presence of the king. She was also the Countess of Niebla until she transferred the title to her eldest child.

After her return from France she devoted herself to the family archives, containing eight million documents, the oldest signed in 1228, which prompted her inquiring mind to promote further controversies. In Africa versus América and No Fuimos Nosotros (It Wasn't Us) she argued that African or Phoenician merchants reached America before Columbus in 1492.

Relations with her children broke down over her decision to establish a foundation to protect the integrity of the estate. On her deathbed she married her secretary Liliana Dahlmann, taking advantage of same-sex laws introduced recently by the Spanish Government. She first met the young German woman at a wedding in 1983 and they had been inseparable since. Ms Dahlmann told El País newspaper last week: “She had no concept of sin at all, she was a free spirit.”

The duchess is survived by her civil partner, two sons and a daughter.

Luisa Isabel Álvarez de Toledo, duchess, social activist, writer and historian, was born on August 18, 1936. She died of pneumonia on March 7, 2008, aged 71