(Bloomberg) ? Peter Lewis, an ex-HSBC Holdings Plc executive bringing a ?5 million ($8.7 million) gay bias claim against the bank, yesterday dismissed as ?ludicrous? allegations he harassed a male colleague by committing a private sexual act in a gym shower cubicle.

Lewis, HSBC?s former global head of equities trading, was fired by the bank after a co-worker complained that Lewis had exhibited ?unwanted behaviour of a non-verbal sexual nature? while staring at him in an adjacent shower stall in November 2004, according to documents submitted to a London industrial tribunal.

Those allegations would have ?received the ridicule that they deserved? if they had been about a heterosexual employee, the openly gay former executive told the court on the first day of witness testimony.

?I was dismissed from the firm for reasons and beliefs which were not based on facts or evidences but for `preferences? which resulted from innuendo, assumption, false stereotype and homophobia,? Lewis said in a written statement. His claim is the largest of its kind brought to a tribunal in the UK and the first from a high-profile financial professional. The case is likely to turn on whether HSBC responded to the complaint against the former executive in the same way that it would have dealt with a complaint brought by a straight employee, employment law experts said.

?Employers need to take appropriate steps to investigate all allegations of harassment and to conduct those investigations fairly,? said Mark Hunt, head of employment at Reed Smith. ?Whether an employer has done this is the key issue to be determined in cases such as this.?

HSBC denies wrongdoing and says Lewis was dismissed after a thorough investigation of his conduct and a disciplinary hearing. David Burnett, the chief operating officer of HSBC?s investment banking unit, was involved in the investigation, according to the court documents.

?The claimant was not treated any less favourably in any material respect and his sexual orientation had no influence whatsoever on these decisions,? lawyers for HSBC said in arguments submitted to the court.

Lewis joined the lender in September 2004 after serving as global head of client trading at Societe Generale SA.

He testified he has been in a monogamous partnership for ten years and finds the idea of casual sex with a stranger ?repulsive?.

?It was well known in the financial community and the City of London that I was gay,? Lewis said. ?I was also aware that it was still considered unusual enough within the investment banking and particularly the trading area for my sexuality to provoke a considerable amount of interest and debate. The situation has improved but discrimination and homophobia in the financial services industry has not been eliminated.?

Lewis claims that he began receiving ?threatening and abusive? phone calls from a person he believed to be an HSBC employee, some using a derogatory term for a homosexual, around four weeks after starting his new position.

He says that while he did work out at the company health club on the night of the alleged incident, ?at no stage whilst I was in the changing room area did I behave as was subsequently alleged against me nor did I behave in a manner that could be so construed by anyone?.

HSBC human resources executives met with Lewis five days later and told him the details of the complaint against him.

He was suspended one day later, according to tribunal documents.

A second allegation from another employee, claiming that Lewis had once removed his towel in the gym steam room while appearing to be ?in an excited state? was also added to the case against him, according to the former executive?s witness statement. He was dismissed from the bank in December 2004 for gross personal misconduct and his appeal of the decision was rejected in February of the next year.

?I cannot even begin to describe the hurt, distress and damage that have been caused to me,? Lewis yesterday testified.

?I was genuinely excited to be joining a firm which I had believed to uphold the highest standards of fairness, honesty and integrity.

?However, that faith was dashed on the rocks of ignorance and prejudice.?

Regulations banning discrimination on the grounds of sexual orientation came into force in Britain in December 2003, after the government implemented a fair employment directive from the European Union.

Other financial professionals have settled their cases out of court, including Sid Saeed, a former Deutsche Bank AG vice president who claimed to have suffered severe health problems as a result of racial and homophobic abuse from colleagues.

Lewis?s case is expected to last ten days.