Chris Keates said teachers could be unfairly criminalised
Teachers should not be prosecuted for having affairs with their sixth formers, a union chief has said.
NASUWT general secretary Chris Keates said it was an "anomaly" that a teacher who had sex with a pupil aged over 16 could go on the sex offenders register.
She told ITV's Tonight programme the law was wrong because a teacher could legally enter a relationship with a sixth former at another school.
Child protection professionals have criticised her comments.
In 2001, the law was changed to make it illegal for teachers to engage in sexual activity with pupils at their school aged under 18.
But Miss Keates said the abuse of trust law had gone too far in cases where the relationship started after a pupil had reached the age of consent.
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Teachers are in a position of trust, breaching that trust is wrong
Keith, Ely, United Kingdom
She said: "If a teacher has a relationship with a pupil at the school at which they teach, it could be an 18-year-old pupil in sixth form, then that teacher can be prosecuted and end up on the sex offenders register.
"Clearly there have to be appropriate disciplinary sanctions in the school where a teacher works to make sure that inappropriate relationships don't develop.
"But it does seem a step too far, when there has been a consensual relationship, to put that person on the sex offenders register when, in fact, they could have a perfectly legitimate relationship with an 18-year-old at another school."
Later, Miss Keates told the BBC the NASUWT had raised the anomaly as soon as the legislation was drafted and that it advises members to keep relationships professional at all times.
"Any teacher or adult who works in schools who sexually abuses a pupil of any age should face the full legal consequences," she added.
Since 1991, 129 teachers have been prosecuted for relationships with pupils but a Sheffield University study suggested as many as 1,500 intimate relationships develop every year.
NSPCC policy advisor Zoe Hilton said: "The law is very clear that if a teacher abuses his or her position by forming a sexual relationship with a pupil they could be prosecuted and this remains the case even if the child gives their consent.
"The law is, quite rightly, there to protect children."
To Sir With Love: Tonight will be broadcast on ITV1 at 2000 BST on 6 October.