Police, fire crews and paramedics were were called to Romsey Conservative Club, which houses the MP’s office, at 11.20am on Tuesday.
The building was evacuated and forensic officers were sent to the scene, but the substance was deemed “not hazardous”.
According to Sky sources, the substance was a white powder, sent in a letter and opened by a member of Ms Nokes’ staff.
“There is no risk to the public,” a police spokesman added.
A cordon was placed around the club while police investigated, but it was later lifted.
The alert came a day after Ms Nokes admitted some Windbrush generation Britons incorrectly identified as illegal immigrants may have been deported.