Spain’s socialist government is finally to fulfil one of its key promises when the remains of General Franco are exhumed from the austere splendour of the Valley of the Fallen and transferred to his family mausoleum outside Madrid.

If all goes to plan, the 1.5-tonne slab that has covered the dictator’s tomb will be lifted at 10.30am on Thursday and the coffin removed and flown by helicopter or taken by road to Mingorrubio-El Pardo municipal cemetery.

Franco’s remains have had pride of place in the basilica, which is 40 miles (64km) north-west of Madrid, since his death in 1975.

Although the Valley of the Fallen and its 150-metre (490ft) cross ostensibly commemorate all those killed in the Spanish civil war, for many people, it serves only to glorify Franco and his four-decade dictatorship.

The government said the dictator’s remains needed to be moved as they could no longer “remain in a public mausoleum that exalts his figure”, adding that the removal would “symbolically close the circle of Spanish democracy”.

Profile Who was Franco? Show Born in 1892 in Ferrol, Galicia, Francisco Franco Bahamonde was a Spanish general and politician who ruled over Spain as head of state and dictator under the title Caudillo between 1939 and 1975. He and other officers led a military insurrection against the Spanish democratic government in 1936, a move that started a three-year civil war. A staunch Catholic, he viewed the war and ensuing dictatorship as something of a religious crusade against anarchist, leftist and secular tendencies in Spain. His authoritarian rule, along with a profoundly conservative Catholic church, ensured Spain remained virtually isolated from political, industrial and cultural developments in Europe for nearly four decades. The country returned to democracy in 1977 but his legacy and place in Spanish political history still sparks rancour and passion.

For many years, thousands of people commemorated the anniversary of his 20 November 1975 death in Madrid's central Plaza de Oriente esplanade and at the Valley of the Fallen mausoleum. And although the dictator's popularity has plummeted, the 2019 exhumation of his body has been criticised by Franco's relatives, Spain's three main rightwing parties and some members of the Catholic church for opening old political wounds. Photograph: Photo 12/Universal Images Group Editorial

The move has been bitterly opposed by the Franco family, the foundation that bears his name, and the Benedictine abbot who presides over the basilica.

The family mausoleum where Franco’s remains are to be taken. Photograph: Pierre-Philippe Marcou/AFP/Getty Images

But after months of appeals and legal arguments, Spain’s supreme court gave permission for the exhumation at the end of September.

On Thursday morning, 22 members of the Franco family will gather in the basilica, along with Spain’s justice minister, Dolores Delgado, in her role as first notary of the kingdom.

A canopy has been erected to cover the grave and guard against any attempts to film the exhumation. Those present will be checked for electronic devices to make sure there are no images or sound recordings of the exhumation.

Last Saturday, a journalist dressed as a monk and equipped with a climbing harness was arrested at the valley, allegedly while trying to gain access to the site to set up cameras.

Once the heavy slab has been lifted using hydraulic jacks, the wooden coffin will be inspected to see how well it has endured 44 years of burial and checked for possible water damage. The coffin was surrounded with lead and concrete to protect it against underground streams.

Franco’s body is further protected by a zinc box within the coffin, which is expected to be in good condition. If necessary, the zinc box containing the remains will be placed in a new coffin.

The exhumation phase could take between one and three hours, after which the remains will be blessed by the prior, Santiago Cantera, and the coffin carried out of the basilica by family members.

The Valley of the Fallen. Photograph: Manu Fernández/AP

If flying conditions are suitable, an air force helicopter will transport the body, Delgado and one Franco family member to Mingorrubio-El Pardo cemetery, a 15-minute flight from the valley. Airspace over the route will be closed to traffic and drones.

If it is too foggy or windy, the coffin will be driven in a hearse, which could take 40 minutes.

A private family service will be held inside the Franco mausoleum, where the dictator’s widow, Carmen Polo, was buried after her death in 1988. At the family’s request, mass will be said by Ramón Tejero.

The priest is the son of Antonio Tejero, the Guardia Civil lieutenant colonel who led the failed 1981 military coup that sought to seize control of the country after its return to democracy following Franco’s death.

According to the government, the exhumation and reinterment will cost about €63,000 (£54,000).

Francisco Franco, centre, gives a salute. Photograph: AP

Twelve years ago, the previous socialist government of José Luis Rodríguez Zapatero introduced the historical memory law, which sought to help the country come to terms with its past and assist people in finding and exhuming the bodies of relatives killed in the civil war.

About 100,000 victims of the conflict are thought to lie in the unmarked graves where they were buried during the conflict. Spain is believed to have more mass graves than any country except Cambodia.

The Valley of the Fallen, partly built by the forced labour of political prisoners, is Spain’s largest mass grave and holds the bodies of more than 33,000 people from both sides of the civil war.

In July last year, the Spanish government announced plans to establish a truth commission to investigate crimes against humanity committed by the Franco regime.

The government, led by the prime minister, Pedro Sánchez, said it would compile a census of the victims of the civil war and the ensuing dictatorship, and also open an estimated 1,200 mass graves.