Armed police remain locked in a stand-off with a man who is said to be in possession of dangerous items including petrol and combustibles.
Up to 80 people have been evacuated from addresses in the immediate area in west London as a precaution.
Scotland Yard said police were called on Saturday, shortly before 1am, to reports of concerns for an occupant and hazardous articles inside the address at Wood End Lane, Northolt.
Police negotiators and armed officers remain at the scene. So far the man has not engaged with authorities and all options are being considered.
The Met says its priority is to bring the incident to a safe conclusion - something which may take some time.
Police are concerned for the man's wellbeing and believe he may currently be experiencing mental health issues, which could be exacerbated by police action or may make his behaviour unpredictable.
Chief Superintendent Paul Martin, in charge of policing Ealing, said: "Our aim is to bring this to a safe conclusion for everyone. I know that our operation has been ongoing for some time now and that it is causing disruption and inconvenience to local residents and the community of Northolt.
"I would like to thank them for their continued patience and cooperation.
"I can understand why people may be feeling frustrated or concerned. Their local officers are out on duty today to speak to local people, keep them updated and understand any concerns that they may have.
"The operation will continue for as long as is necessary and I would like to stress that we are doing all we can - using all our tactics - to end this incident safely."
There is no suggestion that this incident is terror-related or linked to any other ongoing police operation or investigation.