The family of a murdered British spy said today they were "deeply upset" over claims about his private life and suggested the security services may be behind a smear campaign.

Gareth Williams's body was discovered in a holdall in the bath of a flat in Pimlico, central London, on Monday. Police believe he may have been dead for two weeks.

Williams, 30, worked at GCHQ, the eavesdropping and security centre, was days from completing a year-long secondment at MI6 and, it has emerged, was a regular visitor to the US national security agency that intercepts and analyses foreign communications.

However, reports that police had found evidence linking him to a male escort and that bondage equipment was found at his flat were challenged by his family.

William Hughes, 62, a Plaid Cymru councillor on the Isle of Anglesey county council and a cousin of Williams's mother, Ellen, said there was no evidence the allegations were correct.

He said: "I don't see any evidence of it. It never crossed my mind that Gareth was that sort of person. He left home at a young age and what happened in his private life was his business.

"When you have these rumours in the papers, it is most distressing. It is heartbreaking that he has died so young and his family have enough on their plate without having to read these stories.

"Gareth's parents are not doing well at all. They are in a state of shock and struggling to come to terms with what has happened. They have seen what has been in the papers and they are very, very upset about these untruths."

Hughes said it was possible the government or another agency may be trying to discredit Williams, who had been due to return to a rented flat in Cheltenham, where GCHQ is based.

The Daily Mail reported that Williams flew to the NSA, the Pentagon's listening post, up to four times a year. He returned from his last trip a few weeks before he was found dead. The paper said US intelligence officers were poring over Williams's work and personal life to see whether the circumstances of his death endangered US national security.

Tests are being carried out to determine whether he was asphyxiated, and whether drugs or alcohol were in his system. The tests are expected to take weeks.

Williams left school at 15, got a first-class degree in maths at 17, and a PhD at Manchester University. His former maths teacher, Geraint Williams, said: "He was the best logician and the pupil with the fastest brain I have ever met. You only had to say things once, that's why he was so successful."

Investigators suspect Williams may have known his killer as there was no sign of forced entry at his top-floor flat. Police have begun examining his mobile phone and financial records as well as CCTV cameras from streets and businesses surrounding his home.

His flat has been the subject of a fingertip search amid fears that top-secret work material could have gone missing. Officers broke down the door of his flat on Monday afternoon when attempts by government officials to locate him through his former landlady failed.