Nottingham Castle will not reopen its doors to visitors until March 2021 'at the earliest'.
A spokesperson for the castle says the decision has been made "reluctantly" due to the national lockdown and significant increases across the county.
A provisional reopening date has been set for March 29 next year. It was initially scheduled for February 8.
The city council has been working with the castle on a transformation project worth £30m, which includes adding an adventure play area to the grounds.
Officials say they are "extremely disappointed" to make the decision after a lot of hard work has gone into the project.
Nottingham Castle Trust chair Ted Cantle said:“We are very pleased with the way the Castle project is shaping up, but do have to recognise the reality of the Government's Covid-19 regulations.
"However, a seven week delay will not dent the enthusiasm for the opening of this exciting scheme, which will create many new jobs and with the story of Robin Hood attract people from all over the world.
"Following the £30m investment and a successful opening, Nottingham Castle will again become the pride of the city.”
The new castle experience aims to combine interactive, virtual technology including Robin Hood folklore alongside high-quality art exhibitions, to bring Nottingham's 1,000-year history to life.
Nottingham City Council Portfolio Holder for Leisure and Culture, Councillor Dave Trimble, said: "Given the current position with the Covid pandemic, it’s understandable that this decision has had to be made.
"Fortunately, it doesn’t affect the tremendous progress we have already made despite Covid-19 to transform the Castle into a world-class heritage attraction for our city.
"We will continue to closely monitor the position in order to ensure the Castle can re-open its doors as quickly as possible.”
