The state of properties owned by Labour MP Jas Athwal is “unacceptable”, Sir Keir Starmer said.
The Prime Minister said it was “not good enough” for the MP to allow tenants to live in flats infested with ants and with black mould on the walls.
Sir Keir said Mr Athwal had to take action to put it right “as quickly as possible”.
Mr Athwal, the MP for Ilford South in east London, owns a rental portfolio including 15 residential and three commercial properties in the capital, according to Parliament’s register of MPs’ interests.
A BBC investigation found black mould in some of the properties, with dirty and dark communal areas, fire alarms hanging loose from the ceiling, and ant infestations in a number of the seven flats in one block.
Sir Keir said: “It’s unacceptable for any landlord, and I’ll be really clear about that it doesn’t matter whether it’s a Labour MP or anybody else, it’s unacceptable.
“What’s happened in this case is this MP has now recognised that, is taking the necessary measures to put it right.
“The sooner that’s done, the better, but I’m not going to pretend to you or anybody else that this is in any way acceptable.”
Asked if he would strip the Labour whip from the MP if the problems were not resolved, Sir Keir said: “It has to be put right. He’s taking action to put it right, we need to do that as quickly as possible.”