The monarch was a 21-year-old princess when she married the dashing Lieutenant Philip Mountbatten at Westminster Abbey on November 20 1947.

Their enduring relationship has lasted the longest of any British sovereign, and Philip has been at the Queen's side throughout the decades, supporting her as she devotes herself to her role as head of state.

While the duke has retired from public duties, the Queen has an official engagement on the morning of their anniversary on Tuesday - when she visits the Royal Institution of Chartered Surveyors in London to mark its own 150th anniversary.

Last year saw the Queen and Philip toast their milestone platinum wedding anniversary with a black-tie party for 100 family and friends in the state apartments at Windsor Castle.

This year's anniversary is expected to be low key.

There is no traditional gift, jewel or colour associated with 71st wedding anniversaries in the UK.

With two royal weddings, two royal babies and one on the way, 2018 has already been a year of celebration for the Windsors.

The Queen and the duke's grandson the Duke of Sussex wed American former actress Meghan Markle in May, while their granddaughter Princess Eugenie married tequila brand ambassador Jack Brooksbank in October.

Prince Louis - the monarch and Philip's sixth great-grandchild - was born in April, followed by a seventh great-grandchild, Lena Tindall, in June.

Harry and Meghan's first child - an eighth great-grandchild for the Queen and Philip - is due next year.

The Queen and the duke have also just celebrated the 70th birthday of their eldest child, the Prince of Wales.

At their 1947 wedding Princess Elizabeth walked down the aisle, dressed in an ivory silk Norman Hartnell gown, decorated with 10,000 seed pearls, glittering crystals and an intricate four-metre star-patterned train, to marry 26-year-old Philip, who was fresh from serving in the Royal Navy in the Second World War.

Within five years, the princess had acceded to the throne.

Their 71-year-union is seen as a key source of stability within the monarchy.

Together they have celebrated the Silver, Golden and Diamond Jubilees of the Queen's reign, and faced ups and downs over the years including the breakdown of three of their four children's marriages, and the backlash which followed the death of Diana, Princess of Wales.

On their golden wedding anniversary in 1997, the Queen paid a touching tribute to her husband, saying: "He has, quite simply, been my strength and stay all these years."

Philip praised the Queen for her abundance of tolerance.

"I think the main lesson that we have learnt is that tolerance is the one essential ingredient of any happy marriage," he said.

"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."

Australian Associated Press