Queen Elizabeth II and Prince Philip are celebrating their 70th wedding anniversary with a private dinner at Windsor Castle.

The couple's four children, Charles, Anne, Edward and Andrew, along with the Queen and Philip's grandchildren, including Prince William and Prince Harry, will be raising a glass to the longest marriage in British royal history.

Even pregnant with her third child, Kate, the Duchess of Cambridge, will also be there alongside the monarch's great-grandchildren, Prince George and Princess Charlotte.

This milestone anniversary is markedly different from previous public celebrations of the diamond wedding anniversary in 2007, which included a public service of Thanksgiving.

But the quiet event is at the couple's request as the pair are said to be against traditional celebrations following the deaths of a number of close friends in recent years.

It remains unknown whether Prince Harry's American actress girlfriend, Meghan Markle, will attend the low-profile celebrations after Queen Elizabeth II reportedly gave the 36-year-old divorcee her seal of approval.

The Queen reportedly met Meghan at an intimate tea party earlier this year, and the couple's other grandchildren Peter Phillips, Zara Phillips, Princess Beatrice, Princess Eugenie, Lady Louse Windsor and James, Viscount Severns are also expected to be at the low-key dinner, while their great-grandchildren Isla Phillips, Savannah Phillips and Mia Tindall could also make an appearance if they are old enough to stay up.

The evening marks 20 years since the longest-reigning monarch famously paid tribute to her 96-year-old husband as her strength and stay.

In 1997, the Queen said of Philip, “He has, quite simply, been my strength and stay all these years, and I, and his whole family, and this and many other countries, owe him a debt greater than he would ever claim, or we shall ever know.”

Twenty years ago, Philip also revealed, “I think the main lesson that we have learned is that tolerance is the one essential ingredient of any happy marriage.

It may not be quite so important when things are going well, but it is absolutely vital when the going gets difficult.

You can take it from me that the Queen has the quality of tolerance in abundance.”