Sir Keir Starmer has said that under a Labour government, the prime minister, chancellor and deputy prime minister would all publish their tax returns.
The Labour leader vowed to add greater transparency in publishing the internal register of ministerial interests, with the party considering using an opposition day debate this week to give Tory MPs a vote on the matter.
It comes as No 10 grapples with the controversy surrounding Tory chair Nadhim Zahawi, as well as questions about former prime minister Boris Johnson and the appointment of Richard Sharp as BBC chairman.
Downing Street has said that Mr Sunak will publish his own tax return “in due course”.
Sir Keir told the Sunday Times: “I think it’s pretty simple: those in charge of taxation can’t also be seeking to avoid it.
“I’m mystified why the prime minister couldn’t just say that this week. I see no reason why the prime minister shouldn’t publish their tax return as a matter of course. It happens elsewhere already.”
“And if we are serious about restoring trust, I think anyone appointed as chancellor or deputy prime minister should do the same,” said Sir Keir.
He added: “Trust in politics matters. It’s why I said from the start that if I were fined over Durham, I’d resign. It’s why we are committed to cleaning up Westminster. The public need to know that the swamp is being drained.”
Earlier, the Labour leader used a speech at London Labour’s conference to tell members not to be complacent about the battle to win the next general election.
Sir Keir’s party is widely tipped to do after the series of scandals and controversies that have plagued successive Tory administrations.
“The Tories will never give up on power,” Sir Keir told members. “That’s not who they are. So don’t let up. No complacency. Fight for every vote.”
Meanwhile, it has been reported that Mr Sunak received informal advice in October that there could be a reputational risk to the government from Mr Zahawi’s tax affairs.
Government officials are said to have given the new prime minister informal advice as he drew up his cabinet in October, regarding the risks from a HMRC probe – first revealed by The Independent – that was settled while he was chancellor.
According to The Observer, Mr Sunak was informally warned back in October that the tax issue involved a significant amount of money.
But Downing Street has strongly denied the claim that Mr Sunak was warned of the risk. A No 10 spokesperson said: “These claims are not true. The prime minister was not informed of these details, informally or otherwise.”