That is dramatically lower than the 2.6 meters assumed earlier from NASA's Shuttle Radar Topography Missions data.

The region, which spreads over 40,577 square kilometers (over 10 million acres) and is currently home to 21.49 million people, is one of the largest and seemingly lowest delta plains in the world.

"While ongoing land subsidence increases the rate of relative sea level rise, the sediment load of the Mekong River to counterbalance relative sea-level rise with sediment accretion on the delta plain is dwindling due to upstream dam construction and decreased hurricane activity in the Mekong catchment," the study published on Nature Communications warned.

It estimated that at its current rate of subsidence the delta could see seawater covering the 0.8 meters within 57 years, requiring over 12 million people living in areas that would be inundated by the sea following a rise of one meter to relocate.

That figure is double the number predicted earlier of more than five million following satellite data analysis.

For this study, the team from Utrecht University acquired a large dataset of elevation points from a detailed topographical map of the delta referenced to Vietnamese geodetic datum with mean sea level measured at the Hon Dau, a tide gauge in northern Vietnam, as zero datum.

The team then created a new elevation model for the delta based on these elevation points, and compared it to previous satellite data.

At a conference held by the Ministry of Natural Resources and Environment in June experts said one of the biggest problems the delta faced was saltwater intrusion.

This has been blamed on upstream hydropower projects in the Mekong River, which flows through China, Myanmar, Laos, Thailand, and Cambodia before draining in the sea in Vietnam.

The upstream dams retain mud and sand and along with that alluvium, a report by the environment ministry said.

In a study released last December the World Wildlife Fund said the Mekong Delta is subsiding by 2.5 centimeters a year.

The study, which WWF conducted in collaboration with Vietnam Rivers Network and People's Aid Coordinating Committee, also blamed the upstream dams for lowering the volume of sediments flowing to downstream areas.

Marc Goicho, program advisor at WWF, told Reuters in September last year that sand mining in rivers was depriving many low-lying Asian deltas, the Mekong Delta being a typical example, of the sediments they needed to maintain themselves, thus raising the risk of worsening land loss to rising sea level.