'They were friends before their relationship blossomed. But Peter and Lindsay have gone their separate ways as the relationship simply ran its course,' a source told the Sun.
A friend of the couple added to that Peter's work commitments means he's been travelling 'non-stop'.
'Peter and Lindsay made the difficult decision to separate as they were spending less time together with work and family commitments. Peter's business commitments and work in F1 meant he has been away and travelling non stop,' the source said.
'It obviously came as a shock as most people thought they were absolutely rock solid, it's very sad. Peter has been travelling all over the world and has been incredibly busy and Lindsay is based up in Scotland, so it's been hard for them to see each other.'
The late queen's grandson, who lives on his mother's Gatcombe estates, does not get money from the royal grant and works as head of partner acquisition for the rights agency CSM Sport & Entertainment and as an ambassador for ISPS Handa, as well as various other endorsement gigs.
In June, a friend of the King's nephew told Hello that Peter has always been 'extremely close' with Charles and that 'it meant a lot' that he was able to bring Lindsay to
The source told the outlet: 'He is in a great place at the moment and could not be happier in either his private or personal life.'
The friend added the pair look 'so natural' in each other's company, adding they are 'incredibly happy' together.
'Lindsay really is a breath of fresh air and it's clear to see both Lindsay and Peter are loving life,' the friend continued.
Peter and Canadian-born Autumn announced they were separating in 2020, with the divorce finalised in 2021. They share two daughters, Savannah, 13, and Isla, 12.
Meanwhile, stay-at-home mother Lindsay parted from businessman husband of 11 years Andrew, at around the same time last year that Peter announced his divorce.
Autumn is still has a good relationship with some members of the family. In the summer, she was pictured at a charity golf tournament with Zara, Peter's sister.
Meanwhile rumours of a romance between Peter and Lindsay first emerged when he made a visit to see her in March 2021 - raising questions around whether he had broken lockdown restrictions.
Eight months later, he showed he was serious about his new relationship when he brought Lindsay as his date to his nephew Lucas's christening.
The couple were photographed in a car together ahead of the gathering.
Peter, the eldest child of Princess Anne and her first husband Captain Mark Phillips, then took the highly significant step of introducing Lindsay to his late grandmother Queen Elizabeth II, who was said to have been 'delighted'.
Lindsay was introduced to Her late Majesty at a shooting party at Windsor and the encounter was described as 'warm' by a source.
A spokesman for Mr Phillips confirmed at the time that Lindsay had accompanied him to the joint christening of Zara and Mike Tindall's son, Lucas, and Princess Eugenie and Jack Brooksbank's son, August, in Windsor.
Lindsay, a mother-of-two, knows Mr Phillips and his sister, Zara, from their school days at £45,000-a-year Gordonstoun.