The constituency office of immigration minister Caroline Nokes has been reopened after tests showed a substance found in an envelope was harmless.
Police sealed off Romsey Conservative Club in Hampshire at about 11:20 BST, while emergency workers in white protective suits spoke to those inside.
The cordon was lifted at about 14:40 after police said there was no risk to the public.
Ms Nokes confirmed a staff member discovered the suspicious substance.
The Romsey and Southampton North MP was not in the building at the time.
In a statement, police said: "A cordon was put in place as a precaution while tests were carried out to establish what the substance was.
"Following those tests we can confirm that the substance is not hazardous and there is no risk to the public."
Police said some officers would remain on the scene in Market Place while inquiries continue.
Ms Nokes issued a statement thanking emergency services for dealing with the situation "so swiftly and professionally".
She said: "This morning staff in my constituency office opened an envelope which contained a suspicious substance.
"They followed guidelines, evacuated the office and informed the police immediately.
"At this stage it appears this was a hoax and I am relieved that all those involved are safe and well."