A gay investment banker at HSBC has begun a £5m court case over his dismissal, which he claims was motivated by his sexuality.

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 who is believed to earn 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.”

A spokesman for the bank told the Evening Standard that Mr Lewis was dismissed “after a lengthy disciplinary proceeding”.