King Charles and Queen Camilla previously denied an invitation to the wedding of Lady Tamara Grosvenor, daughter of the Duke of Westminster, and Edward van Cutsem in 2004, which was dubbed society wedding of the year.

The Queen and Prince Philip attended the ceremony but noted absentees were the then Prince of Wales and Camilla Parker Bowles, following a "bitter disagreement over seating arrangements for the ceremony" as reported at the time by Daily Mail.Charles was to have been seated in the front row but his companion at the time was relegated to seats on the other side of the aisle towards the back as dictated by royal protocol.

According to Richard Eden at MailOnline, a source revealed: "Their Majesties have not forgotten what happened at the wedding of Hugh's sister. There is still a lot of tension that exists."

Royal author Sarah Bradford revealed that Queen Elizabeth II was annoyed at how the situation unfolded and informed her son that things couldn't carry on as they were. An insider at the time shared: "Her Majesty is a stickler for protocol. It would be inappropriate for Mrs Parker Bowles to sit beside Prince Charles since she has no official standing", reported the Mirror.

Camilla was 23 when she met the eligible British heir, 22, at a polo match in 1970, just one year after Charles’ investiture as the Prince of Wales. Charles and Camilla announced their engagement on Feb. 10, 2005, and married at a civil ceremony held at Windsor Guildhall, on April 9, 2005.

Neither Queen Elizabeth nor Prince Phillip attended the civil ceremony. They did, however, attend a blessing by the Archbishop of Canterbury in St. George’s Chapel at Windsor Castle and the reception.

It is believed the late Queen bowed out of the wedding ceremony because of her role as head of the Church of England, which discourages divorce.

Writing in The Telegraph in April 2005, Andrew Alderson explained the real reason behind the late Queen’s refusal to attend the civil ceremony. He wrote: "The Queen has let it be known that the reason she will not be attending the wedding of Prince of Wales is because she is putting her duties as the head of the Church of England before family feelings."

However, she did turn up to the later religious service conducted by the Archbishop of Canterbury in St George’s Chapel and Queen Elizabeth also hosted a lavish wedding reception for more than 700 guests in Windsor Castle afterwards.

Addressing the party of 800 people, the late monarch kicked off her highly-anticipated speech with a nod to one of her lifelong passions – horses.

Referring to Aintree Racecourse at the annual Grand National steeplechase, she said: "They have overcome Becher's Brook and the Chair and all kinds of other terrible obstacles.

"They have come through and I'm very proud and wish them well. My son is home and dry with the woman he loves."