Mark and Heather, a former art teacher, are believed to have met at a riding clinic in Auckland while the former army captain was still married to Princess Anne.
The pair were alleged to have indulged in a one-night stand at a nearby hotel. Infamously, he left his riding boots outside the door to help her find the right room.
When Heather found out she was pregnant, a shocked Mark tried to convince her to have an abortion.
Adamant that she would keep her unborn baby, Heather went through with the pregnancy, affectionately giving her little girl the nickname ‘Bunny’.
Meanwhile, Mark secretly paid his one-time lover a meagre $12,000 a year in child support, but refused to have anything more to do with his secret daughter.
When the payments trailed off, Heather sought to secure a better deal for Felicity, however Mark refused to be named on her birth certificate.
In 1991 a court ordered DNA test proved he was the father.
Defending her decision to make the matter public and threaten Phillips with court action, Tonkin said in a previous interview, “I am doing what I am doing for my child. I hope and pray Mark will do the right thing and make a proper and legally binding settlement on her.
"I wish I could wake up one morning in the knowledge that the record had been put straight and I don’t have to worry any more.
“Nothing can compensate for the tears I have cried while trying to plan for Bunny’s future, when at any moment I could find myself penniless.
"Bunny throws her arms around me to comfort me, asking why I am sad. But I have never been able to tell her. My ambition is to get Mark's public acceptance of her and to be able to enter his name on her birth certificate," she added.
Felicity, who shares a love of horses with her half-sister Zara, has been left heartbroken by her father’s rejection.
Having learned the truth about her father when she was 8, after completing a school family tree project, the little girl reportedly tried to learn as much as she could about her well-connected family.
According to The Daily Express, the now equine vet kept a book about Mark by her bed and spent hours devouring articles about her British ‘family’.
“She’d have liked a card or something, some sort of recognition,” says Heather’s former boyfriend Jason Fowell.
“She wished she had a father to go to school interviews, that sort of thing. I saw her cry about it.”
“The story heaped shame on the royal family,” says a palace source, adding that none of the royal family have had anything to do with Felicity.
“They just don’t want to get involved and that’s probably the way it will stay,” the source revealed.
In 1999, Mark was quizzed about whether he intended to visit his daughter, with both attending the same equestrian event in New Zealand.
When asked by a journalist if he would meet Felicity, he brusquely replied, “Wrong subject”.
“Like a lot of men who fall to temptation and have a one-night stand resulting in a pregnancy, Mark blamed the woman concerned,” a source told New Idea.
Indeed, during a taped meeting with Mark’s business agent, James Erskine to discuss financial support for Felicity, he told Heather: “If you want to cause a flap, everyone’s just going to deny it,” adding that the “clout” lay with Mark and “when this thing hits the fan, your daughter’s life will be ruined.”
Felicity’s grandmother, Dr Shirley Tonkin, also confirmed that Mark, along with Felicity’s half-siblings Peter and Zara, had never attempted to contact Felicity.
“Zara has never been in touch with her -– nobody has,” she told Daily Mail in 2011.
While Mark and Princess Anne had already separated at the time of the bombshell confirmation that Mark had fathered a love child, the news prompted Anne to start divorce proceedings.
The couple finalised their divorced in 1992, and Anne also kicked Mark out of her country home Gatcombe Park, where he had been living since the couple split.
“This is all the excuse she needed to get Mark out of Gatcombe,” a source told People magazine at the time. “Mark has had it ... Anne’s hand will be strengthened significantly.”
In 1997 Mark married dressage rider Sandy Pflueger, and their daughter Stephanie was born later the same year.
The marriage ended in 2012 after it was reported that Mark had left Sandy for another woman, Lauren Hough, who was 32 years his junior.
In 2015, Felicity married polo player Tristan Wade.
In a surprising twist, Wade is a regular fixture on the polo circuit, and has played at the Royal Windsor Cup in front of the Queen and Prince Philip.
In 2017 the couple had their first child, James.
Sadly, Mark Phillips still refuses to be acknowledged as his grandfather.