A 100-metre cordon has been put in place in Denham after police found a suspicious substance while executing a warrant at a property.
Thames Valley Police said officers were carrying out enquiries at an address in Tilehouse Lane, Denham, near Uxbridge, when the substance was discovered.
Emergency services remain at the scene and specialists from the Explosive Ordnance Disposal Unit (EOD) are assisting while the material is assessed and the area is made safe.
Police have asked members of the public to avoid the area while the operation continues.
The force said there is no wider risk to the public, although a police presence will remain in place while enquiries at the property are ongoing.
No further details have yet been released about the nature of the substance or the reason for the original warrant.
