Hi
Thanks for posting your question to the helpline.
You asked which expression was correct: "May I help you?" or "Can I help you?"
Both are correct; it depends on what you mean. In formal styles, "may" asks permission; so the question is “Will you permit me to help you?” "Can" asks about ability and the question becomes “Am I able to help you?” A clerk in a department store may ask permission to be of service; a passer-by, seeing a car with the bonnet lifted, may enquire about the ability to be of assistance. So choose the phrase to fit the context.
Regards
Workplace English Training Expert