The respected political aide, who mixes in the highest circles at Westminster, said they enjoyed a series of sordid encounters at gay fetish club The Hoist in London.
According to the 53-year-old adviser they had sex in a room of the club, which holds “spanking and punishment” parties, on several occasions between 2003 and 2008.
The following year, businessman Dewani, 32, met wife-to-be Anni, 28, – and is accused of plotting her murder on honeymoon in South Africa in 2010.
Not long after the killing, as suspicion fell on Dewani and police searched for possible motives, he strenuously denied claims he was secretly gay.
The Daily Star Sunday understands it was that denial which prompted the aide, who lives in London, to come forward with his bombshell claims.
Sources close to the South African police investigation say he approached and gave a statement to Scotland Yard in late December 2010.
The Parliamentarian, whose name we have decided not to reveal, said their first meeting was nine years ago.
He is understood to have told detectives that Dewani was a “submissive” who enjoyed S&M and dressing up in leather.
The Hoist’s website says the venue has a “strictly enforced” dress code consisting of leather, uniform, rubber and “full sports kit” outfits.
Membership of the club, which is under railway arches at Vauxhall, south London, is £35.
The aide is believed to have told the Scotland Yard officers the relationship with Dewani continued for five years.
He is the second man to come forward and claim he had a sexual relationship with Dewani.
The other man, Leopold Leisser, a male German escort who advertises his services as “The Master”, has told police Dewani paid him to have sex on three separate occasions.
Leisser is being lined up as a major prosecution witness should Dewani stand trial.
During an extradition hearing in London last year, prosecutors told the court they had a witness who claimed that
Dewani had told him he needed to “find a way out of getting married” and that he would be “disowned” by his family if he attempted to break off his engagement.
On Friday, South African prosecutors were told they will have to wait for the Bristol man’s mental health to improve before he can be extradited for trial.
Last week BBC Panorama broadcast an hour-long investigation into the case, which included previously unseen CCTV footage of Dewani and his wife.
Other film showed him meeting the taxi driver who has confessed to his involvement in the murder plot.
Zola Tongo claimed Dewani offered him cash to arrange Anni’s murder and make it look like a hijacking. He is serving an 18-year prison sentence.
The care homes boss vigorously denies any involvement in his wife’s death.
Last week the Dewani family’s lawyer Charlotte Harris, of Mishcon de Reya, said in a statement: “Shrien has not been charged with any offence in any country.
“Despite his offers at an early stage he has not been interviewed by the South African authorities in connection with any offence.
“Shrien is innocent and has always maintained his commitment to clearing his name.”