Archaeologists in Cambodia have found multiple, previously undocumented medieval cities not far from the ancient temple city of Angkor Wat, the Guardian can reveal, in groundbreaking discoveries that promise to upend key assumptions about south-east Asia’s history.

The Australian archaeologist Dr Damian Evans, whose findings will be published in the Journal of Archaeological Science on Monday, will announce that cutting-edge airborne laser scanning technology has revealed multiple cities between 900 and 1,400 years old beneath the tropical forest floor, some of which rival the size of Cambodia’s capital, Phnom Penh.

Some experts believe that the recently analysed data – captured in 2015 during the most extensive airborne study ever undertaken by an archaeological project, covering 734 sq miles (1,901 sq km) – shows that the colossal, densely populated cities would have constituted the largest empire on earth at the time of its peak in the 12th century.

Evans said: “We have entire cities discovered beneath the forest that no one knew were there – at Preah Khan of Kompong Svay and, it turns out, we uncovered only a part of Mahendraparvata on Phnom Kulen [in the 2012 survey] … this time we got the whole deal and it’s big, the size of Phnom Penh big.”

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The Angkor temple ruins, which sprawl across the Unesco-protected Angkor archaeological park, are the country’s top tourist destination, with the main temple-city, Angkor Wat, appearing on the Cambodian national flag. Considered the most extensive urban settlement of pre-industrial times, and boasting a highly sophisticated water management system, Angkor’s supposed decline has long occupied archaeologists.

The new cities were found by firing lasers to the ground from a helicopter to produce extremely detailed imagery of the Earth’s surface. Evans said the airborne laser scanners had also identified large numbers of mysterious geometric patterns formed from earthen embankments, which could have been gardens.

Experts in the archaeological world agree these are the most significant archaeological discoveries in recent years.