Opposition leader Sam Rainsy has said he believes Prime Minister Hun Sen’s recent push to convert the maps of Cambodia’s border with Vietnam to a scale of 1:50,000 and a UTM-style map projection would not require a constitutional amendment.

Rainsy this week said he supported Hun Sen’s push for France to help Cambodia and Vietnam convert the 1/100,000-scale Bonne-projection maps that the French drew, and on Tuesday night he argued on Radio Free Asia that the new maps would not violate a key article in the Constitution.

“The Constitution does not need to be amended to allow a UTM map with a scale of 1:50,000 to be used. It is only an additional tool to make the map more detailed and accurate. There is no change in the content of the map,” Rainsy said in the radio interview.

“Let me clarify: We would like France to help convert the scale, but not the system. We would like the scale of 1:100,000 to be converted to the scale of 1:50,000,” he said. “The latter version of the map has more detailed information which can make border demarcation work easier and more accurate.”

Article 2 of the Constitution says: “The territorial integrity of the Kingdom of Cambodia shall absolutely not be violated within its borders as defined by the 1/100,000 scale map made between the year 1933-1953 and internationally recognised between the years 1963-1969.”

The issue of the border has long been a tinderbox for the government, with Rainsy’s opposition in 2015 running a devastating campaign accusing the government of ceding territory to Vietnam by using maps other than the 1/100,000-scale Bonne maps. The premier’s recent proposal to redraw the maps still proved contentious yesterday, even within the opposition.

Thach Setha, a former senator and the most prominent opposition member who is Khmer Krom, a term for ethnic Khmers born in parts of Vietnam annexed from Cambodia, said he feared losing land with the projection being changed from the Bonne to UTM method and more detail being added.

“The Constitution limits it to the 1/100,000 scale. If we want to be correct, we have to draw it according to this scale,” Setha said, explaining that converting the maps from the Bonne projection to UTM would necessarily change the content of the maps.

Setha’s view was shared by former deputy prime minister Lu Lay Sreng – formerly a member of the royalist Funcinpec party who has since aligned himself with the CNRP – who said the use of 1/50,000-scale maps differing from the Bonne maps would be necessarily unconstitutional.

“We must amend the Constitution first,” Lay Sreng said, citing the second article. “Don’t let the prime minister do things contrary to the Constitution.” Meanwhile, deputy CNRP leader Kem Sokha’s top adviser, Muth Chantha, in a Facebook post on Sunday also appeared to question the political savvy of supporting different maps, saying another foe of Hun Sen had fallen by agreeing to CPP-backed border policies.

“Prince Norodom Ranariddh was strongly criticised as the person who gave life back to the 1985 border treaty because Funcinpec raised their hands to vote for the 2005 supplemental border treaty,” said Chantha, who was Ranariddh’s top adviser in the late 2000s.

“I don’t understand the border maps issue, but I love my country, and I still have the will to protect Cambodia’s sovereign territory according to the 1/100,000 maps.”

Chantha could not be reached for comment yesterday. Kem Monovithya, the eldest daughter of Kem Sokha and the CNRP’s deputy head of public affairs, declined to comment on the issue, and directed a reporter to the comments made by Setha.

Rainsy did not respond to a request for comment.