US intelligence agencies interrogating the brother of a disgraced Communist official believe he is the most valuable Chinese defector to flee to America, according to two people familiar with some of the intelligence he has provided.

The defector, Ling Wancheng, is the brother of Ling Jihua, the former chief of staff to President Hu Jintao who was formally detained on suspicion of “serious violations” of Communist party rules in December 2014.

The secrets Mr Ling has revealed to US investigators include details on Chinese procedures for launching nuclear weapons, the personal lives of China’s leaders, and arrangements for their security and for the protection of the Zhongnanhai leadership compound in central Beijing, according to one senior retired diplomat and a former leading western intelligence official who received briefings in Washington.