SHANGHAI (Reuters) - The trial in China of a British corporate investigator and his American wife and business partner arrested a year ago after they did work for GlaxoSmithKline PLC will be closed to the public, two family friends with knowledge of the matter told Reuters Thursday. The detention of Peter Humphrey and Yu Yingzeng has sent shockwaves through the foreign business community. For years, the community has relied on investigative firms such as the couple's Shanghai-based company ChinaWhys to better understand the Chinese business environment. In April 2013, GSK employed ChinaWhys to investigate an ex-employee suspected of sending anonymous emails, including the circulation of an intimate video of former GSK China head Mark Reilly with his girlfriend, as well as emails containing allegations of widespread bribery at the British drugmaker. Three months later, authorities detained Humphrey and Yu for illegally buying and selling private information. Chinese authorities have not openly made a link between GSK and the case against ChinaWhys. The couple were subsequently arrested, which typically means the police believe they have enough evidence for the case to be brought to trial. They are being held in Shanghai, but it is not known in which court they will be tried. U.S. consular officials had been informed on Wednesday when visiting Yu that they would not be able to attend the trial, and that the decision to keep the trial closed had been made on the grounds of privacy, according to the couple's family friends, who declined to be identified because of the apparent sensitivity of the case. The trial date has also been pushed back a week to Aug. 7 from July 29, the people said. The U.S. and British consulates declined to comment. CLOSED TRIALS In China, court cases involving state secrets or national security are regularly closed to the public. However, the couple's son said in a statement this case involved neither. "I am very worried that family and consular officials are not allowed to attend my parents' trial. This does not involve state secrets. This does not involve national security," Harvey Humphrey, 19, said in the statement to media. "I am surprised at this decision since China wants to promote openness and the rule of law and I hope that they will let me in." One of the family friends said the defendants' lawyers had been required by the prosecutor to sign non-disclosure agreements. "The client doesn't even know what's going on," the person said. The reason for the non-disclosure agreements was not clear, but underscore the sensitivity of the case. Officials contacted by telephone at the Shanghai procuratorate and municipal intermediate courts said they did not know about the case. There was no listing of the trial on Aug. 7 or July 29 on an online docket for all of Shanghai's courts. China's foreign ministry did not immediately respond to faxed requests for comment. Chinese authorities last month charged Reilly and other colleagues with corruption, after a government investigation found the firm made billions of yuan from elaborate schemes to bribe doctors. GSK's tangled web of problems in China were highlighted in a lengthy draft report seen by Reuters. The report had been prepared by ChinaWhys and presented to GSK on June 6 Last year. The report into the origin of the video and emails did not reach a conclusion as to who was behind the actions. Humphrey previously worked for Reuters in Asia, Eastern Europe and the Balkans, according to the website of ChinaWhys. (Reporting by John Ruwitch; Editing by Ryan Woo)