Prince William has chosen to have his brother Harry at his side as his best man on his big day, St James's Palace said today.
Kate Middleton's maid of honour at the wedding on April 29 will be her sister, Philippa, a spokesman said.
The couple have selected four bridesmaids and two page boys - two of whom are the Prince's godchildren.
The bridesmaids are Lady Louise Windsor, seven, Margarita Armstrong-Jones, eight, Eliza Lopes, three, and three-year-old Grace van Cutsem - Prince William's goddaughter.
The page boys are William Lowther-Pinkerton, 10, and eight-year-old Tom Pettifer. Tom is the godson of Prince William.
Bridegrooms in royal weddings traditionally choose a "supporter" instead of a best man. However, Prince William opted to have a best man rather than a supporter, a St James' Palace spokesman said.
When royal bridegrooms select their supporters there is usually more than one, with a chief supporter taking the main role, but because William has only chosen one companion on his wedding day he decided to use the term best man.
The Prince of Wales chose his brother Prince Andrew to be his main supporter when he married Lady Diana Spencer at St Paul's Cathedral in 1981.
Harry, who is training to fly Apache attack helicopters, is likely to have the role of planning William's stag do.
On the wedding day he could be tasked with looking after Kate's wedding ring. He may also have to give a speech at the private reception following the ceremony.
Harry said he was "delighted" about the engagement when it was announced.
He said: "I am delighted that my brother has popped the question! It means I get a sister, which I have always wanted."
The brothers are only two years apart and have been close since their mother died in 1997.
After the engagement was announced, bookies had tipped Thomas van Straubenzee - one of William's closest friends - to be a supporter.
Mr van Straubenzee, also known as "Van", met the Prince at Ludgrove Prep School and he accompanied William on his first official tour to New Zealand. William and Harry are patrons of a memorial fund dedicated to Tom's brother Henry who was killed in a car crash in 2002.
Miss Middleton's younger sister, Philippa, 27, has been given the role of maid of honour.
Prince William's future sister-in-law, known as Pippa to her family, is no surprise appointment as she is often seen by Kate's side and they are said to have a close relationship.
She has received substantial media attention since her sister began her relationship with the Prince and is often photographed out with Kate.
After graduating in English from Edinburgh University, Pippa worked as a party planner and is rumoured to be a "gatekeeper" for her sister.
She recently raised eyebrows over an interview conducted shortly after the engagement announcement, with some accusing her of using her sister's wedding as an opportunity to promote the family's mail-order company, Party Pieces.
It has been rumoured that Kate's parents, Michael and Carole Middleton, would like their younger daughter to play a more prominent role in their business in the future.
The St James's Palace spokesman said details of the bridesmaids' dresses and the page boys' uniforms would be made available on the wedding day.
The bridesmaids at the wedding of the Prince of Wales and Lady Diana wore ivory satin dresses with gold sashes and lemon slippers. The page boys wore 1863 naval uniforms.
The wedding details were disclosed on the same day that it was announced the couple would be paying a visit together later this month to the university where they met.
Prince William is also to take on the role of patron of St Andrews University's 600th Anniversary appeal.
The couple will return to the Fife university on Friday February 25 to launch its 600th Anniversary programme.
On Thursday February 24 the couple will attend a naming ceremony and service of dedication of the Hereford Endeavour lifeboat at Trearddur Bay Lifeboat Station on Anglesey, where RAF rescue helicopter pilot William is based.
The visits are not the first the couple have made together since their wedding announcement. They attended a Teenage Cancer Trust concert in Norfolk in December.