A radiation leak at the site of North Korea’s massive bomb test is “inevitable”, a Chinese nuclear weapons expert warned, after authorities in China reported that the mountain where the explosion took place had collapsed.

China’s Earthquake Network Centre said a second 4.6 magnitude ‘earthquake tremor’ happened eight minutes after Sunday’s initial explosion, which measured 6.3 magnitude.

Authorities in China said the tremor could be a result of a “cave in”, a potentially devastating collapse of the mountain at the Punggye-ri test site, where North Korean tests its weapons underground.

Wei Shijie, who worked on China’s nuclear weapons programme from 1964 to 1990 and is often quoted on the subject in Chinese media, said: “When I heard there could have been a collapse, it made me so concerned because it means the problem is quite serious and a nuclear leak is inevitable.”

“But even if the mountain did not cave-in, a leak will definitely happen,” added Mr Wei.