Is it a mortal sin not to fast on required days?

I’m not talking about fasting before Communion, but during Lent and on whatever other days were are asked to fast, is it really a mortal sin to not fast? I always assumed it was just beneficial as opposed to necessary.

Recommended reading:

Annual Lent Fight by Jimmy Akin

(Note: Scroll to section titled “Penance in General.”)

DISCLAIMER: The views and opinions expressed in these forums do not necessarily reflect those of Catholic Answers. For official apologetics resources please visit www.catholic.com.