Sir Keir Starmer previously insisted he would sack any minister who broke the rules amid a row over Angela Rayner. The Tories highlighted a quote from the Prime Minister in a biography by Tom Baldwin published last year.
It comes as the Deputy Prime Minister faces a probe into whether she broke the ministerial code after she admitted to underpaying tax on a new property. The biography quotes Sir Keir as saying: "People will only believe we are changing politics when I fire someone on the spot. If a minister, any minister, makes a serious breach of the rules, they will be out."
Asked if it was someone as "vital to his project" as Chancellor Rachel Reeves, he replied: "It doesn't matter who it is, they'll be sacked."
Earlier today, the Prime Minister declined to say whether he would sack Ms Rayner, who is also the Housing Secretary.
Sir Keir said "of course" he will act on the findings of his independent standards adviser following Ms Rayner's acknowledgement that she did not pay enough stamp duty on a flat she bought in Hove this year.
But he would not be drawn on whether he would fire her if Sir Laurie Magnus finds that a breach of ministerial rules took place.
In an interview with the BBC, he said: "There's a clear procedure. I strengthened that procedure. I am expecting a result pretty quickly.
"I do want it to be comprehensive... and then of course I will act on whatever the report is that's put in front of me."
Ms Rayner has been under mounting pressure in recent weeks after reports emerged she had saved £40,000 in stamp duty on a property purchase in Hove by not paying the higher rate reserved for additional home purchases.
She referred herself to independent adviser Sir Laurie and said she made a "mistake" in paying the standard rate, based on legal advice she received at the time.
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