Sir Keir Starmer has repeatedly refused to say if he will sack Deputy Prime Minister and Housing Secretary Angela Rayner if his standards adviser concludes she broke the ministerial code.
Rayner referred herself to Sir Laurie Magnus after she admitted underpaying stamp duty on an £800,000 flat in Hove.
She has said the "mistake" was the result of incorrect legal advice which failed to "properly take account" of her circumstances.
Sir Keir told the BBC he would "of course act" on Sir Laurie's report which he expected to be "comprehensive" and delivered quickly but would not be drawn on whether Rayner would be fired.
Tory leader Kemi Badenoch said the prime minister did "not have a backbone".
