Nationally, child protection experts have condemned a teachers' union boss for saying staff who have affairs with pupils over 16 should not be prosecuted.

Last night Chris Keates, who is general secretary of the National Association of Schoolmasters Union of Women Teachers, told ITV Tonight that teachers shouldn't be imprisoned if the pupil involved is over the age of consent.

Today Preston headteacher David Fann, Lancashire's executive member of the National Association of Headteachers, said society has high expectations of teachers and his union took the view that parents had a right to expect those morals to be upheld.

He said: "As an association we are uncomfortable with this suggestion from the NASUWT and believe it is a dangerous proceedure to be encouraging.

"Society expects us to behave in a responsible manner.

"Although having a relationship with a pupil over 16 is not breaking the letter of the law, in terms of the moral code of society they have crossed it."

He added: "Although there have been times when I have questioned it (the code], we have to be bastions of that moral code because that is what society expects of us.

"If she is saying that people in this situation should not be placed on the sex offenders register I do have some sympathy, but the fact is society has high expectations of people such as teachers, police and the church."

The programme was broadcast on ITV1 last night.