The Army and Navy flyover in Chelmsford could be closed for up to 18 months.
The structure, which was declared "sound" by Essex County Council on Tuesday, will shut for "essential repair works" from 23 July.
A Temporary Prohibition of Traffic Order states the closure will be in place for 21 days - but work could continue for up to a year and a half.
In recent months, local residents have raised a number of concerns about the safety of the flyover.
Councillor Mark Springett, Liberal Democrat for Moulsham Lodge, who said the flyover was "riddled with concrete cancer" after chunks started crumbling away, shared news of the closure on Twitter.
He called for residents to be told "the truth" about the safety of the structure after several cracks and signs of corrosion were photographed earlier this week.
@BBCEssex So now they close it for safety reasons, for anything up to 18 months! At Tuesday's Full Council Cllrs Bentley said it was safe, now it's not, we need to know the truth after these cracks and corrosion were photographed this week. #fb pic.twitter.com/ybrF8IXIp4— Cllr Mark Springett (@MarkSpringett) July 12, 2018
The flyover was built as a temporary structure in 1978 and was expected to be in place for around four years.
Earlier this week Kevin Bentley, cabinet member for highways at the Conservative-run county council, insisted the flyover was "in good condition", despite some sections of the concrete cladding beginning to "deteriorate".
Today, he said: "We have no concerns about the safety or structure of the Army and Navy flyover and I want to assure residents the flyover is safe.
"However, we have a planned period of routine maintenance scheduled which will begin on 23 July for three weeks, focusing on the parapet connections and main traffic control and sign system.
"I would like to apologise in advance for any disruption residents may face while this work is undertaken."