James Waiters, 38, taught at High Storrs School, Sheffield, between 2007 and 2015.
A misconduct hearing was told the relationship began shortly after her A-levels in 2015 and the couple are still together.
The National College for Teaching and Leadership panel heard that Mr Waiters had taught the pupil, who is now aged 20, at the school from September 2014.
In December that year, he began to tutor her at home on the request of her mother and continued to do so for three months.
The student began a period of study leave in May 2015, before leaving school in 2015, with the pair beginning a sexual relationship in August.
The relationship was reported to the headteacher, who flagged up the matter to Sheffield City Council in September. And following disciplinary investigation meetings, Mr Waiters was suspended in November 2015, before he was eventually dismissed.
Mr Waiters argued that until the end of July 2015 the relationship was strictly professional. He indicated that he and the student had grown close during the course of their private tutoring arrangement, denying there was anything untoward and stating that her parents were present at all times.
Mr Waiters said the relationship only began to develop after the end of the school term after the pupil agreed to take part in a soul band gig, which he was helping to organise. He said they began to spend an increasing amount of time together, which included band rehearsals, local runs, tennis and a day trip to Manchester. He admitted that the relationship became sexual in or around the last week of August 2015.
The panel heard the relationship stopped in early September 2015 when the student’s parents objected to it; however, they have since relented and, having received their consent, the relationship began again at the end of October 2015 and has continued.
The panel concluded the relationship was inappropriate whether or not the student had formally come off the school roll and that Mr Waiters had “allowed his professional boundaries to become blurred”.
It also found that Mr Waiters lied to the headteacher about the relationship after “panicking” and was therefore found to be dishonest, despite the teacher eventually admitting he had regretfully made “a big mistake” in lying, taking immediate steps to rectify the situation.