King Charles and Queen Camilla have an incredibly strong marriage with the help of two things, according to Her Majesty’s sister, Annabel Elliot. Ahead of the couple's landmark 20th wedding anniversary on Wednesday, Ms Elliot spoke to The Daily Mail and praised the King and Queen's relationship.

In the interview, the Queen’s sister not only touched on how the couple have always supported one another, but also looked at the attributes which have made Camilla so successful in her role. Elsewhere, Ms Elliot also opened up on the two factors in the King and Queen's relationship that have kept it so strong for such a long period of time — friendship and humour.

She said: "Theirs, as any marriage should be, is a huge friendship. The key thing is that there is not really anything competitive between them. She is not trying to steal his thunder in any way at all.

“He is the King. And she will do her best to support him and be totally loyal to him.”

Camilla’s sister also said the Queen has a “really good sense of humour” which has also strengthened her marriage to Charles.

She said: "The humour. It’s been such a bond between them. And even if they argue, they will always laugh."

King Charles and Queen Camilla will be celebrating their 20th wedding anniversary in Rome this Wednesday.

The couple will be treated to a lavish state banquet held in Rome on their special day as part of their four-day state visit to Italy.

Their Majesties will be joined by prominent figures from Italian society at the black-tie dinner hosted by Italy’s President Sergio Mattarella, which will be held at the Quirinale - a former Italian palace and one of the official residencies of the President of the Republic.