"We can confirm the existence of the tape," a GSK spokesman said.

Chinese prosecutors originally wanted to charge Humphrey and Yu with several offences, including some relating to illegal business operations.

But they decided to drop all of them except for one of illegally buying information, a source close to the family said.

Although each faced only one charge, they risked being jailed if found guilty, the source added.

Prosecutors had made Humphrey and Yu's lawyers sign a non-disclosure agreement preventing them from revealing certain information to the couple, the source said.

Both had their own lawyers, who came from the same law firm. But the source would not say which law firm they were from.

Humphrey and Yu are co-founders of ChinaWhys, a Shanghai-based risk consultancy that was closed down by the authorities in August.

They were detained while conducting investigations on behalf of GSK.

Reilly, who was then heading GSK's China operations, had asked Humphrey to find out the identity of a whistle-blower who had made corruption allegations against GSK in China, the source said.

"Reilly suspected the source was a Miss X who was fired from GSK, but wasn't sure," the source said.

Humphrey asked Reilly if the whistle-blower's allegations were true, and the GSK chief had told him they weren't, the source said, adding: "Peter wouldn't have taken the case unless he got the assurance from Reilly."

In May, the Ministry of Public Security accused Reilly of working with other GSK executives to bribe doctors and health officials, and of covering it up. The British citizen has been prevented from leaving China.

Reilly was apparently filmed having sex with his Chinese girlfriend at his Shanghai home.

The whistle-blower sent the video in an e-mail to GSK's top management, including chief executive Andrew Witty, in March last year, and of London reported.

The whistle-blower's e-mail also contained allegations of corruption in GSK's operations in China.

GSK's China unit, run by Reilly, hired ChinaWhys to look into the security breach, reported.

"The investigation by the Chinese authorities remains ongoing and we are cooperating fully with this investigation," the GSK spokesman said. "The allegations that have been raised are deeply concerning to us.

"We have committed significant resources to find out what happened in China, including an independent legal review."

Britain's Serious Fraud Office is conducting a criminal investigation of GSK's activities in several countries.