Is it legal to tutor on a student visa?

[lewis12]

If you're a non-EU citizen studying a Masters degree (not English Literature or related to language) on a student visa, can you teach English in education centers and language academies or tutor privately? Do you need an additional certificate or permit?

Assuming:

• You're qualified (TEFL certificate)

• You'll work less than 20 hours a week to still be able to study effectively