Is 'Il n'y a pas de quoi' formal or informal French?

Il n'y a pas de quoi! is informal even though it is the complete form of Pas de quoi! The courtesy translates into English as "Don't mention it!" when a "You're welcome!" is a response to "Thank you!" The pronunciation will be "eel nya pad kwa" for the complete form and "pad kwa" for the truncated version in French.