Sean Pengse, a former government minister under the Lon Nol regime and an outspoken critic on Cambodia’s border issues with Vietnam, has died in France at 82.

Pengse passed away after a long illness in the early hours of Tuesday morning, according to a statement issued by the Cambodia Border Committee, of which he was president.

Renowned for his engineering mind, he carved a controversial figure in Cambodia, where his vocal campaigns for the Kingdom’s territorial integrity often clashed with the ruling Cambodian People’s Party and the border treaties the government had signed with neighbouring Vietnam.

Pengse was married to the late Vann Daret, the niece of the late acclaimed architect Vann Molyvann. Pengse is survived by his four children, Serge, Theodore, Eric and Ketty. Born in Kampong Cham province in 1936, he attended high school at Lycée Sisowath in Phnom Penh before travelling to France, where he earned a diploma in engineering between 1959 and 1965.

Returning to Cambodia, he served as the director of mining services at the Ministry of Industry, Mines and Handicraft during the Sangkum era under Prince Norodom Sihanouk.

Shortly after the Lon Nol coup of 1970, he was named commissioner general of energy, mines and sea before being promoted to minister of industry, mines and Handicraft in 1973.

The following year he headed a Cambodian delegation involved in negotiating border disputes with Vietnam – a role that acted as a cornerstone for what would later become his legacy as a fierce advocate for territorial sovereignty.

Before the fall of Phnom Penh to the Khmer Rouge in 1975, Pengse fled to France, where he resided for much of his life.

He founded the Paris-based Cambodian Border Committee in 1999. Through the CBC, he repeatedly argued that Cambodia had lost land to Vietnam due to border treaties that were signed in the 1980s, when the Kingdom was still occupied by its stronger neighbour.

At a critical point in 2005, Pengse was charged in absentia for defamation over an interview he gave on Beehive Radio, criticising Prime Minister Hun Sen for signing a supplementary border treaty with Vietnam. Beehive President Mam Sonando, who conducted the interview, was swiftly jailed.

Again in 2010, Pengse claimed controversial border markers were planted illegally, because a 1985 treaty used as a basis for the border delineation was cancelled, and further said the government failed to consult landowners who lost their land as a result – both claims the government has denied.

The border remains a hot-button issue in Cambodia today. Former opposition lawmaker Um Sam An, a close associate of Pengse, is currently serving a two-and-a-half-year jail term for “incitement” for claiming on Facebook that the government had ceded territory by using incorrect maps to draw the border.

Dy Kareth, a former journalist during the Lon Nol regime and a close friend to Pengse, said he was both passionate and “furious” about the border issue, and he regarded the treaties as “a violation of international law”. But to those who knew him well, “he is a gentle, honest, generous man . . . with high morality”, Kareth added.

The Cambodian Watchdog Council also put out a statement mourning “the loss of a map-drawing expert and a valuable human resource to the Cambodian people, who spent half his life trying to protect Khmer sovereignty”.

Rong Chhun, a former union leader and member of National Election Committee, said Pengse had “sacrificed tremendously” for the sake of Cambodia’s land and maritime borders. “We are sad and shocked because we have lost an important person for the country,” he said. “Whenever he talked about the border, the government was not happy toward all of his statements. They rejected and reacted strongly back to him.

“All the ideas he stated should be taken into account and they should not be perceived as thorns in your eyes, because it was to help the nation to survive.”

Indeed, Cambodian People’s Party spokesman Sok Eysan, while offering his personal condolences to Pengse’s family for their loss, said the party was unlikely to issue a letter expressing similar sentiments. “We have different ideas, because he holds liberal ideas and he accused the CPP of being communist, and that accusation is in contrast to the reality,” Eysan said.

Sean Pengse’s body will be cremated on January 29 and the seventh day of his funeral will take place on February 3 at Purthivong pagoda in France.