Adding 'de' to a Paternal Surname in Spain
 

A correspondent on an English talk-board has stated that in Spain it is illegal to prefix your paternal surname with an unauthorised 'de'. I'm fairly certain that I read somewhere that this could easily be done on payment of a nominal fee at a registry office.

Can anyone confirm or deny this, please?

Both sound rather unlikely to me. My understanding is that prepending "de" is the way a woman who wants to take her husband's surname can do it.

I think you must check with your local office of the "registro civil". In many countries you can legally change your name or surname by paying a fee to the registrar's office. :)

Some time ago many people added "de" and "y" to keep a history of their family names. It might be still possible.