Emergency services have carried out a controlled explosion at an old industrial estate in Uxbridge.
London Fire Brigade (LFB) have been at the scene of the old Trimite Site, on Ashley Road, off Cowley Mill Road, since 10.07am on Wednesday (May 11).
Anyone within 100m of the site was evacuated ahead of the explosion, which is likely to have been heard throughout the surrounding area.
Chemicals were found after they disturbed during building work on the site, and the area was evacuated as a precaution.
An LFB spokesperson said: "We have two fire engines and specialist equipment vehicles at the scene.
"At 10.50am, a safety cordon of 100m was established.
"A controlled explosion is due to be carried out at approximately 11.30am."
It is not yet known how many people were evacuated from the scene.
Police were called at 10am to the construction site where construction materials were thought to dangerous.
Officers managed cordons at the scene and left at 12.05pm.