The retrial of a US-based Chinese missile expert convicted of leaking state secrets has been postponed, in violation of legal procedures, his sister said. Hua Di, 64, a Chinese-born US green card holder and once a researcher at the Centre for International Security and Co-operation at Stanford University, California was jailed for 15 years on November 25 by the Beijing Municipal Intermediate Court for leaking state secrets. Hua appealed to the Beijing Municipal Higher People's Court, which overturned the ruling on March 16 and ordered that the case be tried again at the Intermediate Court. It was believed to be the first time the initial verdict in a political case had been overturned. 'According to China's legal procedure, the retrial needs to be opened within two months of the order. In other words the retrial deadline should be May 15,' said Wu Sabao, Hua's sister, who lives in Beijing. 'Despite the fact the retrial deadline is long overdue, I was recently informed that Hua's retrial would be postponed as prosecutors need more time to clarify some information,' Ms Wu said. Hua, a mainland military expert, has cancer. His sister's requests that he be let out for medical treatment have been denied. 'It's torture that the trial is dragging on while Hua is not allowed to receive medical treatment,' said Ms Wu. 'Hua's cancer has spread during two years without treatment. Authorities should, for humanitarian reasons, let him receive outside treatment. 'Hua's lawyer was allowed to visit him two weeks ago and had relayed Hua's renewed request for permission to leave jail for cancer treatment,' she said. Authorities in Washington have repeatedly voiced concern over Hua's case. But Ms Wu said a recent stabilising of Sino-US relations might not help her brother's case. Evidence in his trial included an article he published in 1992 for a Stanford journal on the mainland's missile development. Hua's lawyer has argued that what he wrote was material previously published on the mainland or overseas. Hua was arrested in January 1998 when he attended a funeral on the mainland, just four days ahead of US Defence Secretary William Perry's visit to Beijing. He previously worked at the Central Commission on Military Science and was involved in China's missile programme for 24 years. He sought political asylum in the US after the Tiananmen Square massacre.