Well-wishers have begun to arrive in Windsor for the wedding of Princess Eugenie and Jack Brooksbank.
Some 850 guests are expected, including X Factor judges Robbie Williams and wife Ayda Field, singer Ellie Goulding and model Cara Delevingne.
Another 1,200 members of the public selected by ballot will listen to a live broadcast in the castle grounds, and thousands will watch from outside.
Many came on Thursday to bag spots for the newlyweds' procession at midday.
The wedding is being screened in full on ITV from 09:25 BST as part of a three-hour special of the daily magazine show, This Morning.
Princess Eugenie, who is ninth in line to the throne, is the daughter of the Duke of York and Sarah Ferguson, and the granddaughter of the Queen.
She and drinks executive Jack Brooksbank will tie the knot at St George's Chapel - where Prince Harry and Meghan Markle married in May.
The pair went for a final rehearsal at the venue on Thursday, with photographers capturing shots of a smiling Eugenie in her car.
In an interview for This Morning, Eugenie admitted pre-wedding nerves were "creeping up".
She also described being "all butterflies" when they first met, while skiing in Switzerland in 2010.
"I think I rang my mum that night and said 'I've met this guy Jack'... I remember being like 'I really, really like this guy, I really want him to like me too'," she said.
She realised the feeling was mutual when he gave her a "huge windscreen-wiper wave", she added.
Mr Brooksbank described his future wife as a "bright shining light".
"She's been incredible, she has the ability to do a million things at once in her brain, including working as well as organising everything to do with the wedding," he added.
Who will be there?
Among the celebrities expected to share their special day are supermodel Cindy Crawford, David and Victoria Beckham, singer James Blunt and his wife Sofia Wellesley, and George and Amal Clooney
Members of the Royal Family are expected to begin arriving at Windsor Castle's Galilee Porch at about 10:25 BST, with the ceremony starting at 11:00.
The Duke and Duchess of Cambridge's children, Prince George and Princess Charlotte, will once again take on pageboy and bridesmaid roles.
The other bridesmaids are Mia Tindall - daughter of Zara and Mike Tindall - Mia's cousins Savannah and Isla, Eugenie's goddaughter Maud Windsor and Robbie Williams' daughter, Theodora.
The other pageboy is Louis de Givenchy, six, the son of friends.
Eugenie's sister, Princess Beatrice, is maid of honour. Mr Brooksbank has chosen his brother, Thomas, as best man.
And fans of the Royal Family will be joining in the spirit from outside the castle walls.
Karen Levet and Kim Russell, wearing matching flower crowns in red, white and blue, travelled from Cambridge.
"We missed Harry's wedding so we thought we needed to be at this one," said Ms Levet. "There's a really happy atmosphere here."
Her friend added: "I know people must think we're mad but we love the Royal Family and wanted to be here for the big day, so we're happy to be here this early!"
What time is it happening?
07:00 - castle doors open to the invited public
08:30 - guests start arriving at St George's Chapel
10:32 - bride's mother - Sarah, Duchess of York - and Princess Beatrice arrive
10:35 - groom and best man arrive
10:52 - Queen arrives
10:57 - bride arrives, escorted by her father, the Duke of York
11:00 - service begins
12:00 - bride and groom depart for carriage procession
Italian tenor Andrea Bocelli will perform two pieces during the ceremony, which will also feature music from members of the Royal Philharmonic Orchestra.
They will also hear a personal prayer written by the Archbishop of York, Dr John Sentamu.
Dean of Windsor David Conner will officiate and give an address.
He described the Queen's granddaughter and her fiancé as the "perfect couple" after giving them the traditional pre-marital guidance.
"They are very natural with each other, they obviously love one another but they also have a lot of fun together and it's been a real pleasure to work with them," he said.
The Queen is hosting an afternoon reception afterwards in St George's Hall at the castle. An evening party will take place at the York family home, the Royal Lodge, in Windsor Great Park.
The families are paying for the private aspects of the wedding.
However, republican campaigners say taxpayers face an estimated £2m security bill.
The campaign group Republic argues that as Princess Eugenie, the ninth in line to the throne, does not carry out public duties, the families should foot the bill.
Buckingham Palace says this is a matter for the police.