Kevin Boyle, 52, who was claiming unfair dismissal at Croydon Employment Tribunal, also complained that two teachers were conducting an affair on the school premises and that a nun, thought to be a lesbian, was counselling girls.

Virgo Fidelis Convent senior school in Upper Norwood, south London, is contesting the case. It has denied accusations of bullying of staff by the deputy head, who was later exposed for holding a bogus PhD, sexual harassment of a woman teacher by a female head of department, cronyism and a refusal by the headteacher, Sister Bernadette Davey, to investigate.

Mr Boyle, who spent a little over two years working at the school, claims he was sacked after writing to Croydon council, Southwark diocese and the leaders of Sister Bernadette’s order in France.

The tribunal was told that his letter described Sister Bernadette as being “disastrous” for staff and pupils. He wrote: “Her office is a black hole where everything dies.”

He told the tribunal that she refused to investigate serious allegations properly, many involving the deputy head, Kris Sodhi. He said Mr Sodhi surrounded himself with yes-men, bullied staff and gave an unadvertised job to his lodger.

The tribunal heard that Mr Boyle had claimed there were rumours that Mr Sodhi was having an affair with another teacher in bedrooms above the all-girls Catholic school’s library.

“A member of staff was in the library while the springs of the bed upstairs made an unholy rhythmic twanging,” Mr Boyle said.

He also wrote that one young nun, who was suspected of being a lesbian, was “unstable” but was allowed to counsel vulnerable schoolgirls. She took one girl on regular car trips at the weekend and allegedly told other members of staff some girls “fancied her”, the tribunal was told.

Mr Boyle said he thought that Sister Bernadette refused properly to investigate a sexual harassment complaint brought by a female teacher in June 2001. He claimed the teacher had left the school in tears five weeks before the end of term, alleging her head of department had made unwanted sexual advances.

Mr Boyle wrote his controversial letter in July 2001. He was suspended that November, and sacked in March last year.

Sister Bernadette told the tribunal that she was upset by the allegations and distressed that she had been accused of “being unable to recognise good working practice if it jumped into the bath with me”.

The school’s head of music had died after falling from a school window in 2000 and Sister Bernadette said she was upset by Mr Boyle’s suggestion that she and other senior members of staff had driven the teacher to suicide.

She called Mr Boyle’s claim that Asian teachers were walking into top jobs at the school and eroding its Catholic character “racist”.

The tribunal was told that the then NASUWT representative, Tom Real, confronted Sister Bernadette with complaints from 16 members of staff against Mr Sodhi but she told the tribunal: “I said Mr Real’s actions seemed like a witch-hunt against Dr Sodhi.” She conceded that Mr Sodhi, who is no longer at the school, had a bogus PhD.

Giving evidence, Mr Boyle said he gave up trying to convince Sister Bernadette that Mr Sodhi was a bully.

Asked why he sent the letter, Mr Boyle said: “I had been witnessing serious distress, pupils made ill and calculated bullying. Management was not investigating but demoralising those who raised the issues and showing them who was boss. I’d had enough.”

The hearing concluded and the tribunal will give its decision in writing later.

If Mr Boyle is successful, he will return for a hearing to establish wages due to him. A decision is expected in mid-May.