Five other people, including two children, were taken to hospital as a precautionary measure.
Police received a call at approximately 1.30pm on Sunday about two males – aged 38 and 42 – who were unresponsive at a semi-detached house.
Metropolitan Police detectives are currently treating the deaths as “unexplained”.
“At this early stage, police believe that there was a possible carbon monoxide leak at the address,” the force said in a statement.
“Police in Harrow continue to deal alongside the Health and Safety Executive who have been informed.”
The occupants of the rented multi-occupancy house are thought to have moved in just weeks ago.
London Fire Brigade said units rushed to the scene.
“Two adult males, one adult female, one male child and one male infant have also been treated on site for carbon monoxide poisoning and taken to hospital by London Ambulance Service,” a spokesperson said.
A neighbour, who thought the victims were from eastern Europe, said: “It’s really shocking, very sad, the loss of life. It doesn’t matter how it happens, but it’s really sad.”
The man went on: “It’s a rented property. I know the owner, I’ve met him.
“He was renovating just after Christmas. I don’t know what they did inside, but I think they did carpets, painting.”
The windows of the house were left open on Sunday evening.
Another neighbour said the group had moved in three or four weeks ago: “They are fairly new. As far as I know they’re very quiet.
“They have only been here a short period of time, they have been very quiet.
“I have never spoken to them, they kept themselves to themselves.”
A gas engineer attended the scene to reconnect the gas to neighbouring houses.
Carbon monoxide is a poisonous gas with no smell or taste and breathing it can cause dizziness, nausea and, ultimately, death.