Q. Is it illegal for a hygienist to treat a patient who has not seen a dentist for three years?

There is no true illegality in a situation you describe, although it would almost certainly be regarded as a breach of GDC guidance. The GDC’s view is that three years should be regarded as being a maximum between a dentist’s examination of the patient, as for obvious reasons much could change in that time. Treatment by a hygienist should not exceed that period of time. If a patient has not been seen by a dentist within three years, then a new up to date examination and prescription for the referral is required.

There are of course situations where a new clinical examination might be required after say 2 or 3 months. Much then would depend on the individual patient and it would be sensible to

Related questions:

Q. When a dentist refers to a hygienist can he/she give a prescription for 3/6 monthly or any regular recall appointment valid for three years? Should there be an expiry date? 

Q. If a patient has not been seen by a dentist for three years or less can the hygienist arrange for the dentist to come in at the hygiene appointment, before treatment starts, and the dentist can then write a prescription/referral?