Why Will may miss out

With Prince Harry acting as best man to his brother at his wedding in 2011, it is likely that the Duke of Cambridge will take the role at the wedding in May.

However, according to Town & Country the role of the best man does not exist at a royal wedding with the position known as a 'supporter'.

William is reported to have broken royal protocol by appointing Harry as a best man over the traditional supporting role. 

So technically William is allowed to be by Harry's side at the altar, but he isn't allowed to refer to him as a best man, just a supporter.

Daily Mail.