A leading investor in insurer QBE is demanding the board provide more details on why it sacked chief executive Pat Regan following a complaint by a female employee over inappropriate behaviour, arguing the lack of transparency casts doubt over the decision.

Mr Regan's shock sacking on Tuesday came after a week-long external investigation by law firm Minter Ellison concluded he had made "inappropriate workplace communications" – said to be in the form of text or email messages – that breached the company's code of ethics and conduct.