Some 850 guests are expected, including X Factor judges Robbie Williams and wife Ayda Field, singer Ellie Goulding and model Cara Delevingne. The royal family have also arrived.
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.
Maud Windsor, who is Eugenie's goddaughter and the daughter of Lord Frederick Windsor and his actress wife Sophie Winkleman, is also a bridesmaid.
Fans believe Princess Eugenie is wearing this pretty floral dress.
Mr Brooksbank described his future wife as a "bright shining light".
Senior British royals have begun arriving at St George's Chapel for the wedding of Princess Eugenie.
Prince George will have a pageboy to play with in Louis de Givenchy, 6, whose parents are Zoe and Olivier de Givenchy.
On Thursday, the mother of the bride, Sarah Ferguson, was seen in a vehicle smiling and waving as she arrived at Windsor Castle along with Princess Eugenie.
He then scrambled to hand them over to his best man before being struck down by a cough as the ceremony got underway.
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Famed tenor Andrea Bocelli was among the musicians providing music for the service, including a performance of "Ava Marie". In another difference, the prayers will be led by John Sentamu, the retiring Archbishop of York, while Archbishop Angaelos and The Reverend Prebendary Rose Hudson-Wilkin from the Nave led the prayers at Harry and Meghan's wedding.
David Conner, the Dean of Windsor, will officiate over the vows and give the address.
"The reason that they look very much the same is that Diana's ring was [created] by Garrard & Co in 1981 and Sarah's ring was from the same designer a few short years later in 1986", said Samhan.
The Queen will host a wedding reception at Windsor Castle for the couple and their guests.
But according to campaign group Republic, the policing bill footed by taxpayers could reach up to 2 million pounds (€2.3 million).
The 28-year-old bride, granddaughter of Queen Elizabeth II, is ninth in line to the British throne.