Lincoln's Lucy Tower 458-space multi-storey car park will be closed for longer than expected after it emerged more extensive repairs than initially thought are needed.
But for exactly how much longer is yet to be decided.
The City of Lincoln Council and structural engineers had carried out large scale investigations into the condition of the site before taking the decision to close it for repair works in March for an anticipated 20 weeks.
However, now works to replace concrete and steel reinforcement from the middle decks have started , exposing the steel reinforcement, it has been discovered extra work is needed to secure the future of the structure.
Expert consultants and engineers are now working with the council to draw up plans for the additional work.
This is expected to be completed by the end of the month, when new timescales and costs will be released.
Work has stopped on site until the new specifications have been drawn up.
John Bibby, director of housing and community services at the council, said: "Our initial target of 20 weeks for the completion of improvement works at Lucy Tower car park was based on surveys undertaken by our consultant structural engineers.
"However, once the project began it became increasingly obvious that the site needed more extensive work than had initially been anticipated. This unforeseen issue did not come to light until the concrete was cut away to reveal the steel reinforcement beneath.
"Safety is our first priority and the level of corrosion we found in the steel was worse than had been anticipated. This must be addressed as part of these works in order to ensure the structural integrity of the car park for the future.
"We are currently working with contractors, consultants and engineers to see how we can get the required works completed as quickly and safely as possible and to the highest of standards. It is vital that, once reopened, we don't need to close the building again for the foreseeable future .
"This extra work is inevitably going to cause a delay in the completion of the project, this is unavoidable and we apologise for this inconvenience. We will make the extent of the delay clear to all as soon as we have that detail available.
"Extra car parking provisions put in place during the closure will remain in operation until the Lucy Tower car park reopens."
The car park was built in the 1970s and is constructed primarily of reinforced concrete. Over recent winters the cold weather and salts brought in from the roads has caused corrosion to parts of the car park leading to the decision to close it for essential repairs.
The council will be notifying businesses in the area this week and will keep Orchard Street car park open for public pay and display parking as long as Lucy Tower car park is closed.