Philip Davidson and Andrea Croft, formerly of the Grange comprehensive in Runcorn, Cheshire, were charged with unacceptable professional conduct for allegedly sending and receiving emails containing improper material.

They told the hearing that the culture among science staff at the 1,100-pupil school was of a bawdy nature and that the sexually explicit content of the emails was in keeping with that.

Ms Croft told the hearing, in Birmingham last week, that the emails were part of a staff running joke which was very adult and very crude.

She and Mr Davidson said pupils in their lessons were always working when they used the internet and were never at risk of being exposed to inappropriate material.

They said the school drew pupils from a socially deprived catchment area which made their environment challenging , and that the humour helped to defuse the tension.

Ms Croft and Mr Davidson, who are now working as supply teachers, acknowledged that it was improper to send emails of that nature during lessons.

It is also alleged that they used the internet for personal reasons while supervising children during lessons.

Ms Croft used the internet 25 times in September 2003, nine times during lessons, to look at sites such as Argos and Marks and Spencer as she planned her Christmas shopping.

Barry Carney, the deputy headteacher at the school who monitored the staff use of computers, told the hearing that he was asked by the headteacher to investigate staff internet use in October 2003.

Mr Carney told the disciplinary hearing that over three days in October 2003, he monitored a string of sexually explicit emails which were sent between the two teachers.

He said that between September 1, 2003, and October 16, 2003, Mr Davidson also browsed the internet on 166 occasions, 97 times during lessons.

Mr Davidson said he used the internet as an educational tool to engage difficult pupils in the school’s learning support unit who were at risk of exclusion.

Sheleen McCormack, presenting officer, drew the hearing’s attention to emails written by Mr Davidson to other colleagues, inside and outside school.

She said they were derogatory towards women and pupils who attended the learning support unit which he co-ordinated.

Investigations into the teachers’ conduct were started at the school, but were never completed as both resigned in December 2003.

Ms Croft, who had taught at the school for 13 years, now works as a supply teacher at Heartlands school, in Liverpool.

During the hearing, Mr Carney described her as “a super teacher - among the best in the school”.

The disciplinary committee will reconvene at a later date to consider its verdict.