Britain's Foreign Office said royal officials had sent their regrets to Estibalis Chavez, a 19-year-old fan who has protested outside the country's embassy in Mexico City for 10 days demanding an invite.
Leaders of several Commonwealth nations - and the governor-generals who represent the queen in Canada, Australia and New Zealand - are also thought certain to be included among the guests.
Vickers said many European royals likely to attend are closer in age to William, second in line to the throne, than his father Charles - suggesting the guest list may have a more youthful feel than royal weddings of the past.
William's office said about 1,000 guests will be the family and friends of the prince, his bride and their parents. Roommates who lived with William and Middleton as they studied at Scotland's St. Andrews University, young entrepreneur Guy Pelly and television reporter Tom Bradby - who conducted the only interview with the couple following their engagement - could all receive an invitation.
Around 80 guests will come from charities which William supports, including African wildlife charity Tusk, and Centrepoint, which helps young homeless people.
Thirty members of the armed forces, likely to be William's colleagues from his Royal Air Force search-and-rescue unit, will also attend.