“Flora and Frederik have been spotted getting very cosy and it’s obvious that they’re very close – only last week she was at his home, and they seemed very flirty when they were saying goodbye to one another.
“We hear that they’re in constant contact and they meet up very regularly, and they’re always laughing and smiling when they’re with one another.”
Flora is a former actress and the royal couple attended her 2014 wedding to restaurateur Soren Jessen.
Frederik and Mary and their children - Prince Christian, Princess Isabella and twins Prince Vincent and Princess Josephine - were the picture of unity as they attended Prince Joachim’s 50th birthday on June 7.
There have been reports that Frederik’s one-time party-loving younger sibling – who is sixth in the line of succession to the Danish throne – and second wife Princess Marie have a strained relationship with the King and Queen-to-be, and intend to move to Paris later this year.
Nonetheless, there was no sign of tension between the brothers or Mary and Frederik at the private dinner hosted by the head of Denmark's royal family Queen Margrethe, 79, at Amalienborg.
Just days later, Mary and Frederik were all smiles as they watched their children take part in the Royal Run, a charity race that took place in three different locations across Denmark. Prince Christian, 13, Princess Isabella, 12, and 8-year-old twins Princess Josephine and Prince Vincent, reached the finish line at Amalienborg Palace for the Copenhagen race.
Despite wearing their running gear for the event, and explaining on their Instagram accounts that they would be running, it appeared neither parent managed to take part in the 10km race. Instead, they were spotted congratulating their children and cheering on other participants.
Just over a year ago, Frederik – who acted as regent for two days last week – explained the secret of his relationship with Mary, saying, “The love and the freedom to dare be yourself in the relationship is important all through your life.”
He added: “It will then be filled with good moments and times during which you surprise one another and add to all the good you have created together. Doing something unexpected from time to time can create joy in the relationship.”
Despite this past romantic proclamation, and the relationship between Frederik and Flora being entirely innocent, behind the scenes, the Australian-born Crown Princess will be exasperated by her husband’s latest misstep.
“There’s no doubt that Mary would be unhappy about this friendship of Frederik’s,” says the insider.
“She’s had to read him the riot act a few times over the years about his close female friends – but to flaunt their closeness so openly is very careless of Fred. Mary won’t be happy.”
Crown Princess Mary of Denmark was born Mary Donaldson in Hobart, Tasmania.
Real estate agent Mary met His Royal Highness Crown Prince Frederik of Denmark for the first time at Sydney bar The Slip Inn during the 2000 Olympics, and embarked on a fairytale romance.
Their royal wedding took place at Copenhagen Cathedral in 2004.
Look back on Mary and Fred' epic love story HERE!