Prince Charles, the heir to the British throne, will walk Meghan Markle down the aisle at her glittering wedding to his son Prince Harry after her own father had to pull out because of ill health.
A huge world audience is set to watch the glittering wedding in a chapel within the ancient walls of Windsor Castle, home to the royal family for nearly 1000 years.
"Meghan Markle has asked His Royal Highness The Prince of Wales to accompany her down the aisle of the Quire of St George's Chapel on her Wedding Day," Harry's office Kensington Palace said in a statement on Friday.
"The Prince of Wales is pleased to be able to welcome Ms Markle to The Royal Family in this way," the palace said.
Her father, Thomas, pulled out of the wedding and was reported to have undergone heart surgery this week in a family drama that was played out under the glare of global media attention.
Ms Markle said in a statement on Thursday she was sad he could not make Saturday's wedding.
"Please know how much Harry and I look forward to sharing our special day with you on Saturday," she said.
Her mother, yoga instructor Doria Ragland, was to take tea with Britain's Queen Elizabeth, Harry's 92-year-old grandmother, on the eve of the sumptuous wedding that will be watched by millions across the world.
Buckingham Palace on Friday confirmed that the Queen's husband, Prince Philip, 96, will attend the wedding though he is still recovering from a hip operation.
Harry will marry Ms Markle, a star of the TV drama Suits, in the castle's 15th-century St George's Chapel.
After Saturday's hour-long ceremony, the couple will take part in a procession through the town's ancient streets on a 19th century Ascot Landau carriage pulled by four Windsor Grey horses.
The wedding celebrations, forecast to play out under sunshine and blue skies, will be a sumptuous show of British pageantry.
Supporters hope the union of one of the most popular royals and a glamorous American actor, a divorcee with a white father and an African-American mother, will reinvigorate the monarchy.
Outside the ancient stone walls of Windsor Castle well-wishers on Friday mingled with tourists and swarms of television crews under swathes of British and American flags.
Police are expecting more than 100,000 people to throng the streets outside the castle, the Queen's home west of London and the oldest and largest inhabited fortress in the world, and have said there would be tight security for the event.
Harry, 33, the younger son of the late Princess Diana, has always been a popular member of the royal family.
In Windsor sales of everything from flags and biscuits to tea towels emblazoned with the couple's pictures have been brisk. Some royal fans have slept on the street since Tuesday to catch a glimpse of the newlyweds on Saturday.
"The atmosphere at the moment is wonderful," said Sandra Atkinson, a 54-year-old shop assistant.
"We've sold out of all our mugs, all our tea towels. It's been wonderful for business. We were expecting the Americans to come in, and they have come in."