Brighton and Hove City Council said it was forced to close New England House at short notice after a fire safety report uncovered “intolerable” safety risk within the building.
Businesses in the building, in New England Street, Brighton, say they are losing thousands of pounds in revenue every day as the busy Christmas period looms.
But the council said the closure was “difficult but necessary”.
A spokeswoman said “compartmentation” of the building had been a significant issue, with more than 1,000 inadequacies discovered.
Other issues include a lack of fire protection on staircases which would pose serious risks if the façade of the building were to fail in the event of a fire.
Current fire detection does not cover all of the necessary areas in the building either, the report found.
Councillor Jacob Taylor, cabinet member for finance and city regeneration, said: “I want to offer the council’s sincerest apology to all those affected. This is not a decision the council took lightly or without consideration for the tenants.
“However, given the information we received from the fire engineer’s report, it was a difficult yet necessary action to keep people safe.
Business owners outside New England House (Image: The Argus)
“We completely understand the disruption, confusion and anxiety this temporary closure has caused.
“The council remains dedicated to supporting the tenants of New England House and is working diligently to address the findings and implement the required safety measures to reopen the building as soon as possible.”
One businessman estimated that the building could be haemorrhaging over £250,000 over the three-day closure.
Brighton and Hove City Council previously said it was aiming reopen the building tomorrow, subject to being able to appoint fire wardens.
The council will, however, be limiting the hours in the building.