"I cannot give you any further details other than that but they have received a sort of save the date fax," a spokeswoman for King Constantine, 70, and Queen Anne-Marie, 64, of Greece confirms to PEOPLE.
There is every chance that their eldest son Prince Pavlos and his American-born wife, Princess Marie-Chantal, will be among the guests too. Prince William is a godfather to their eldest son Konstantinos-Alexios, 12.
The full list of those attending the wedding is still being drawn up and finalized by William and Kate and their staff, though it is expected that the royal families of Spain and Greece and Denmark will certainly receive invitations.
Another certainty: Prince Alexander and Princess Katherine of Serbia will be there. They've already RSVP-ed via their Web site, writing that they will be "delighted to attend."
Back in Britain, a royal source confirms that "feelers" have gone out to certain families to ensure there are no scheduling clashes so prospective invitees have a heads-up that they are on the guest list for the big day.