More than 250 staff were evacuated from the Burton Street building at 10.30am and four employees were isolated as emergency services descended on the site.
Employees were kept outside for more nearly five hours while the premises was roped off and specialist police officers tested the substance.
But everyone returned to the offices shortly after 3pm this afternoon after the material was found not to be dangerous.
A spokesperson for Leicestershire Police said: “The substance was found to be harmless and council staff have been allowed back in the building.
“Police officers will be carrying out further inquiries into the package.”
A council spokesperson said: “We have been told the substance is non-hazardous and we are back in the building.”
The council have repeated earlier calls for members of the public not to call at the office today.
The spokesperson added: “We are getting back to business as usual and we are grateful for the patience shown by members of the public.
“We would still urge people not to come to the offices as we had to send some staff home and we are picking up all of our phone calls and messages now.”