Police have urged people living near an industrial estate in Peterborough to stay indoors while a controlled explosion is carried out.
Cambridgeshire Police said staff were evacuated from the Perkins site in Frank Perkins Way shortly after 1.30am following the discovery of an "unstable chemical".
A police statement said: "A team from the Explosive Ordnance Department at RAF Wittering are at the scene and will carry out a controlled explosion at 6.30am.
"The chemical does not pose a risk to nearby residents, however people are being advised to stay indoors as a precaution."
A cordon has been set up around the site and nearby roads are currently closed.
The Perkins site is a manufacturing facility which produces engines for use in construction and agriculture machinery.
Inspector Kieran Mylchreest, from Cambridgeshire Police, said: "The engineering company called us to tell us they'd found this chemical in an unstable state and that they would like some assistance in dealing with it.
"The risk is very, very low to the general public. Those living in neighbouring streets should stay indoors as a precaution.
"We hope to resolve the situation as soon as possible."
A spokeswoman for Cambridgeshire Fire and Rescue Service said a fire engine is on standby at the scene but the service has so far not been involved.