Three people familiar with the first test in July said it stunned the Pentagon and US intelligence because China managed to demonstrate a brand new weapons capability, although they declined to elaborate on the details.

One person said government scientists were struggling to understand the capability, which the US does not currently possess, adding that China’s achievement appeared “to defy the laws of physics”. 

Space and missile experts have been debating the Chinese test since the FT revealed the event at the weekend.

Jeffrey Lewis, a nuclear weapons expert at Middlebury Institute of International Studies at Monterey, said China appeared to have developed a new innovation, but stressed the need to maintain a degree of scepticism.