A British investigator and his American wife who were hired by British drugs giant GlaxoSmithKline are to face trial in China charged with illegally obtaining and selling private information.

Prosecutors in Shanghai have filed charges against Peter Humphrey, 58, and Yingzeng Yu, 61, the official Xinhua news agency said.

The couple's arrest last year coincided with a Chinese probe into allegations that Glaxo staff had funnelled hundreds of millions of pounds through travel agencies to bribe doctors and health officials to boost sales and raise prices.

Their trial, reportedly due to take place next month, will be behind closed doors but the country’s Foreign Office said it was pressing China to allow British embassy staff to attend.

The latest development in the saga saw Mr Humphrey and his wife appear wearing orange vests on state broadcaster CCTV.

They are accused of obtaining information on Chinese citizens by illegally buying it from others, as well as with hidden cameras, or by following people.

It is said that the couple, who ran the Shanghai-based corporate intelligence and consulting firm ChinaWhys, compiled information for reports to clients that were mainly multinational companies such as Glaxo.

In media reports from China today, Mr Humphrey said he had been contacted in April last year by Glaxo's China manager Mark Reilly who wanted him to find out who had leaked bribery allegations to Chinese authorities and senior Glaxo executives.

It has also been reported that the investigators were looking into the origin of a sex tape involving Mr Reilly - who has himself been accused by Chinese authorities in relation to the bribery claims.

Mr Humphrey said on state television that he found out during his investigation that the bribery claims were true, and that he felt "betrayed and used" by Glaxo.

A British Foreign Office spokesman said: "We are aware of Mr Humphrey's case and are continuing to provide consular assistance to him and his family.

"Although the court has announced that it will be a closed trial, we have made clear to the Chinese authorities that we would like Embassy staff to attend the trial and the need for a transparent and fair process. We will continue to do this."

Glaxo had yet to respond to a request for comment.