Gordon Brown has said that Boris Johnson's position will be "untenable" if he is found to have broken the ministerial code in the "cash for curtains" affair.
The Labour ex-Prime Minister said that "clearly" any gifts received from donors or other individuals have to be declared in line with the rules.
Mr Johnson has dismissed as "trivia" allegations that he failed to declare that Tory donors initially paid for the lavish refurbishment of the Downing Street flat.
However, there are early indications that the row is starting to cut through with voters as the polling gap between the Tories and Labour gradually narrows.
Mr Brown waded into the row as the Government tried to dismiss the growing backlash.
"I think if he's [ Boris Johnson ] breached the ministerial code and he's told the House of Commons something that turns out not to be true, that becomes untenable," he told BBC Radio 4's Women's Hour.
"People should not be receiving gifts because they're in Government because you become beholden to people, you become in their debt, you cannot have that and clearly gifts have got to be declared.
"And lobbying, this is another problem we've got. I mean clearly no person should be entering public service simply for private gain, so you've got to have rules that stop people making money by becoming commercial lobbyists for firms, sometimes foreign firms, pressing their own government after they've left office, it's completely unacceptable.
"Public service cannot be a platform for private gain."
He added: "If he (Mr Johnson) would state that these principles were principles that he would uphold, and then publish the information that backs up that he's doing all that then I think people would be satisfied, but you can't really evade a question of wrongdoing by saying 'my mind's on something else', you've got to do the right thing all the time."
Scots Tory leader Douglas Ross said on Sunday that Mr Johnson should quit if he is found to have breached the ministerial code.
But Trade Secretary Liz Truss told Sky News: "I'm not going to answer hypotheticals about what might happen at some future date.
"What the Prime Minister has made clear is he has covered the cost."
The Electoral Commission announced a formal investigation into the row last week, and the Prime Minister has also asked his new sleaze buster Lord Geidt to look at the controversy.
Lord Geidt will look at whether there has been a breach of the ministerial code, which governs the conduct of top politicians.
However the final arbiter of the ministerial code is the Prime Minister, prompting concern about whether Mr Johnson would accept the findings.
He previously overruled his adviser on ministerial standards, Sir Alex Allen, over a probe into allegations of bullying by Home Secretary Priti Patel.
