Police are in a stand-off with a man after receiving reports that he is in possession of dangerous items.
A safety cordon has been put up at a 200-metre radius and 80 people have been evacuated from the immediate area in west London as a precaution.
Officers were called shortly before 1am on Saturday to reports of concerns for an occupant and hazardous articles inside the address at Wood End Lane in Northolt.
Emergency services have not attempted to enter the property due to safety reasons.
A Metropolitan Police spokesman said: "At this stage, police believe there is one man inside the address.
"Whilst it is not known what the man has inside the property, police are treating seriously reports that the items could be dangerous and that attempts have been made by the man to prevent access to the address."
He added: "There is no suggestion that this incident is terrorist-related nor is it linked to any other ongoing investigation or operation."
Armed officers and negotiators are at the scene.