A banker’s £5m compensation claim against HSBC over allegations that the City giant sacked him because he was openly gay is due to start at an employment tribunal in east London today.

An investment banker who claims he was sacked from HSBC because he was gay will have his case heard at an employment tribunal this week.

43-year-old Peter Lewis is openly gay and was sacked by the banking giant for “gross personal misconduct”. The incident in question relates to an encounter between Mr Lewis and another male employee of the bank, which ultimately resulted in a complaint of “sexual harassment”.

It is believed to be the biggest case of its kind to have reached an industrial tribunal since December 2004 when sex discrimination rules applied to gays and lesbians.

Mr Lewis was global head of equity trading and was believed to have earned around £1m a year. His lawyer Alison Downie, of solicitors Bindman and Partners, said her client was dismissed “because HSBC discriminated against him because of his sexual orientation as a gay man.”

The bank denies his claims, a spokesman for the bank told the Evening Standard in October that Mr Lewis was dismissed “after a lengthy disciplinary proceeding”.