Prince Carlos Hugo (RIP)

Prince Carlos Hugo de Bourbon-Parma, who has died aged 80 of prostate cancer, was the last remotely plausible alternative wearer of the Spanish crown. When that cause failed, he became the high-profile leader of a centre-left political party, but that cause failed, too.

In 1957 Carlos Hugo accepted from his ageing father, Xavier, Duke of Parma, the leadership of the Carlist movement. Xavier had received it from his relative Alfonso Carlos, the grandson of Prince Carlos, who believed he should have become King Carlos V in 1830. He had been displaced when his brother, Ferdinand VII, had ensured that the succession would go to any child of his, male or female, shortly before the birth of his daughter, who became Isabella II. The Carlists' dynastic claim, based on an uneasy blend of absolute orthodox monarchism and Catholicism with regionalist populism, gave rise to three civil wars in the 19th century.

Carlism survived in the northern and eastern regions of Spain, and found a natural ally in General Francisco Franco's National movement. This was established in 1937, the year following the start of the civil war, and united the Carlists – who took an active part in the conflict – with other Catholic conservatives and monarchists, and with the Falangists. Once the nationalists had prevailed over the republicans in 1939, the Carlists retained a voice in the leadership, and Franco became the regent for an absent, unappointed monarch.

Young, dynamic and industrious, Carlos Hugo soon showed himself a worthy contender for that post. He embarked on an almost frenetic programme of activity, conferring with the country's leaders and rallying the movement's faithful. His marriage in 1964 to Princess Irene of the Netherlands, sister of the present Queen Beatrix, boosted his chances a little further. The constitutional crisis this caused in the Netherlands, a Protestant marrying a Catholic from a fascist country, only added to the publicity surrounding the event.

But his regal hopes came to nothing. In 1969 Franco designated Juan Carlos de Bourbon, grandson of the last reigning monarch, as the next king, and Carlos Hugo changed tactic. Ably assisted by two of his sisters and progressive Carlists, he began to adopt a critical approach to the regime and to gradually steer Carlism from the far right towards social democracy. By the time Franco died, in 1975, this Bourbon prince was openly propounding socialist policies.

The following year, two of his supporters were killed by gunmen who had teamed up with some of the dwindling band of diehards on the Carlist far right, and the need for democratic electioneering in 1977 and 1979 exposed the dreamland of Carlist politics. Carlos Hugo then abandoned his claim and his party, and went to live in the US, where his friend the economist John Kenneth Galbraith had invited him to teach and research at Harvard University, Massachusetts.

Hugues de Bourbon-Parma was born in Paris, the elder son of Don Xavier and Madeleine de Bourbon-Busset. He studied law at the Sorbonne in Paris, and economics at Oxford University. In 1957, his father declared him Prince of Asturias, and he was rebaptised as Carlos Hugo.

At first he tried to occupy a diplomatic middle ground between stubborn traditionalists, forever harking about Carlist feats in the civil war, and progressive liberals who looked forward to the end of the dictatorship. But Franco would not help him to the throne. As he told his cabinet, "I am fond of him, he is courteous and likable, but he does not seem to me to the right person to be king of Spain."

In 1968, as his hopes dimmed and opposition to the regime become more overt and widespread, Carlos Hugo took the calculated risk of becoming more openly critical, and was exiled.

After establishing himself across the border in south-west France, he developed the Carlist policy of socialismo autogestionario (self-managing socialism). According to this programme, post-Franco Spain would be a socialist confederation of self-governing regions, with the country headed by a popular monarch. The monarch had to act as a moral exemplar for all Spaniards and would have to stand down if citizens thought him or her failing in their regal duties.

Though the Carlist party was regarded by many as a serious contender for regional power in the dying years of the dictatorship, the subsequent elections showed it to be but one of the many small parties that failed to win mass support. The idea of the transgenerational "Carlist people", who quietly kept the faith but would declare their allegiance when needed, proved to be a myth.

In 2003, unexpectedly, Carlos Hugo reasserted his claim to the throne and declared his eldest son, Carlos Javier, head of the dynasty. But while a small Carlist party persists, it now bears witness to a historical memory: the days of real Carlist pretenders have gone.

Carlos Hugo's marriage ended in divorce, and he is survived by two sons and two daughters.