By Denise Roland 

LONDON--A British private investigator freed last week from a Chinese prison accused officials there of withholding medical care to extract a written confession from him, shedding new light on an episode that has highlighted the potential risks for foreigners gathering information in China.

Peter Humphrey was convicted in China last year on charges of purchasing private information on Chinese citizens, in a case tied to a sex tape-and-bribery scandal involving pharmaceutical giant GlaxoSmithKline PLC. In his first interview since his early release from a 2 1/2 year prison sentence, he said he and his American wife, Yu Yingzeng, a business partner at his investigative firm, never confessed to a charge of purchasing the information.

"Neither of us has ever admitted to guilt as charged," Mr. Humphrey said, in an interview at London's Heathrow airport, after touching down in the U.K. late Wednesday. "Because of that I was constantly harassed in prison over signing a thing they called admission of guilt and a statement of remorse."

Mr. Humphrey said that a confession aired on Chinese television soon after he was first detained, in which he and Ms. Yu were shown in handcuffs and orange prison vests, was "heavily cut and pasted and narrated and what we said during those interviews was heavily distorted."

Mr. Humphrey said he was advised by a "civilian doctor" that he needed a prostate examination shortly after he was first detained by Chinese authorities in 2013, but that despite repeated requests of prison officials "every single week" thereafter, he was refused. Mr. Humphrey, who is 59, said that he was diagnosed with a prostate tumor on April 28, a development he branded a "direct consequence" of his mistreatment in detention.

"Some staff deliberately obstructed all of my requests for appropriate medical attention," he said. "These steps were withheld from me in a deliberate effort to force me to sign a confession of crimes, both before and after the trial."

An official at Shanghai Qingpu Prison, where Mr. Humphrey was held, said "his disease was not caused by the detention." He added that the prison could provide relief but didn't have the ability to cure Mr. Humphrey's ailments. "Every prisoner's medical care is guaranteed in our prison," the official said.

At a daily news briefing on Thursday, Chinese Foreign Ministry spokesman Lu Fang said that China didn't mistreat Mr. Humphrey and that his claims are "not true." "Relevant Chinese authorities provided them with the due rights and interests," he said, adding that the couple satisfied court conditions for sentence reductions.

The couple's case highlighted the potential legal risks for foreign companies or businesspeople gathering information in China. Holding or relaying personal information has long been part of due-diligence practices. Businesses do background checks on potential business partners and employees, for example, to help companies avoid running afoul of corruption legislation such as the U.S. Foreign Corrupt Practices Act. But the Chinese government has placed greater emphasis in recent years on protecting personal data.

In their trial last year, prosecutors claimed that the couple broke Chinese law by illegally purchasing private data for their investigative firm ChinaWhys Co. and selling the information to clients that included multinational companies. Mr. Humphrey and Ms. Yu denied that their firm trafficked in personal information, saying they had hired others to obtain personal data when clients requested it. "I never knew that using third parties to obtain information was illegal," Ms. Yu said, according to trial summaries.

In July, U.K.-based GlaxoSmithKline said it had hired Mr. Humphrey to investigate the origin of a sex video taken secretly in the bedroom of the drug company's top China executive. A whistleblower, whose identity was unknown, sent the video along with allegations of bribery to the executive's superiors in London, the company said.

In September 2014, a Chinese court found Glaxo's Chinese subsidiary guilty of bribery and fined the company nearly $500 million. In an interview aired last year by state broadcaster China Central Television, Mr. Humphrey said while jailed that he wouldn't have led the investigation had he known more about Glaxo's situation in China. Glaxo didn't disclose to him any allegations of bribery, he said.

Glaxo wasn't mentioned in the couple's trial. A Glaxo spokesman declined to comment on the interview Thursday. The company said last year that it didn't hire the two to investigate the bribery claims by the whistleblower.

In the interview, Mr. Humphrey listed those he was grateful to, but said Glaxo wasn't among them. He declined to comment about his relationship with the company.

"I'm not prepared at his stage to discuss any of my clients by name. I need time to look at that," he said.

Mr. Humphrey, a U.K. citizen, was sentenced to 2 1/2 years, while Chinese-born U.S. national Ms. Yu received a two-year sentence. Their release last week cut their sentences short by seven months and one month, respectively.

In his interview on Wednesday, Mr. Humphrey said the two had conceded that they possessed "certain information" that they received from third parties. But he defended his conduct, saying, "we never bribed anyone. Our business was focused on the fight against corruption and fraud. We spent year after year rescuing companies from frauds and saving them millions of dollars."

Yang Jie and Laurie Burkitt in Beijing, and

Fanfan Wang

in Shanghai contributed to this article.

Write to Denise Roland at Denise.Roland@wsj.com

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