Keir Starmer said the public should be told if Carrie Johnson is fined for breaching Covid lockdown rules in the partygate probe.
The Labour leader said he agreed with the “general argument” that families should not be dragged into politics.
Downing Street has said it will not disclose the names of anyone who might be fined for parties apart from Mr Johnson, Chancellor Rishi Sunak or Cabinet Secretary Simon Case.
This week the Metropolitan police said it would issue 20 fixed-penalty notices as a result of its investigation into 12 gatherings held in Downing Street while Covid restrictions were in place.
Mr Starmer has said the public is entitled to know if Mrs Johnson is hit with a fine.
He told Sky News: “There’s a huge difference between the situation of the wife of the Prime Minister breaking the rules made by the Prime Minister and any other situation.”
Sir Keir had previously called for Mrs Johnson, if guilty, to be named.
On Thursday, he told reporters: "If Carrie Johnson gets a fixed-penalty notice, then of course it should be made public.”
He added: “My focus is on the prime minister because he is the one who sets the culture, he is the one who oversaw this criminality at his home and his office, he is the one that came to parliament and said all rules were complied with, which is clearly not the case.”
The police is issuing fines in relation to investigations into around a dozen events in Downing Street and Whitehall – including one in the Prime Minister’s flat – while England was under Covid lockdown restrictions.
Following the fines, Justice Secretary Dominic Raab and International Trade Secretary Anne Marie Trevelyan both accepted that coronavirus rules had been breached in Downing Street.
Mr Malthouse, a Minister of State in the Home Office and Ministry of Justice, said it was fair to say the fixed-penalty notices signal that police believed a law had been broken.
Speaking to BBC Breakfast, he said: “A fixed penalty notice means police have a reasonable belief that you’ve broken the law – you still have a right to challenge it if you want.
“Having said that, the police practice is not routinely to release the names of those who receive fixed penalties, and I don’t see why that rule should be waived for those people who may or may not be in receipt of it in Downing Street.”
Mr Malthouse said he had not personally received a fine in relation to the police probe.