A 52-year-old Sikh, who was jailed in 1986 for posing as a doctor and sexually assaulting women, was a guest among many VIPs on the Royal Barge during Queen Elizabeth's River Pageant.
Harbinder Singh Rana was found guilty in August 1986 of five counts of indecent assault, 11 counts of assault causing actual bodily harm and one count of attempted assault, the Daily Mail reported.
Posing as a doctor, he performed internal examinations and administered injections to many women.
Rana served four years in prison, but has now become an important member of the Indian-origin community.
During the afternoon sail Sunday, Rana came in close contact with every senior royal including Prince Philip, Prince Charles, Camilla, the Duke and Duchess of Cambridge and Prince Harry.
The man was invited to join the celebrations by Prince Charles, who he met through his charity work for the Anglo-Sikh heritage trail, a group that promotes Sikh culture in Britain.
Palace sources said Charles was completely unaware of Rana's past, and would never have invited him had he known.
Rana said he had not been asked about his sex crimes before the event.
"I was given the invitation and I attended. The fact that the Prince of Wales invited me clearly shows what I have done for the community since then," Rana told the Mirror.
"I have a relationship with some of Charles's staff, not him -- although I have met him at events. I have made it clear I wasn't representing the Sikh community, I was there because I was very happy to be invited," he said.
He said he never spoke to the queen or any senior member of the royal family while on the boat.
"Harbinder Singh was asked to take part in the pageant as he is a leading member of the Sikh community and someone who has done a lot of charitable work. Guests of the Pageant, including representatives from all major faith communities, spread across a number of key vessels - as with the royal family, who were present on a number of vessels," a palace spokesperson said.
Even at his trial, Rana had protested his innocence, and now continues to say it was a case of "mistaken identity".