New evidence shows that the Great Wall of China is more than two times longer than previously thought, state media reports.

The results of an archaeological survey have found that the historic structure, comprised of a series of fortifications and built over centuries, is 21,196 kilometers long.

This is the first time a definitive figure has been released, Xinhua reports.

A preliminary survey in 2008 had found that the wall only measured approximately 8,850 kilometres.

China's State Administration of Cultural Heritage along with the State Bureau of Surveying and Mapping had been working on the project since 2007. It is part of a national conservation effort aimed at preserving the cultural relic.

Construction of the wall dates back to the 7th century BC. It was built up over centuries to ward off invasions from the north.

The wall, considered the longest man-made structure in the world, is one of the Seven Wonders of the World and was declared a UNESCO World heritage site in 1987.