Historic England said it had “serious concerns” about proposals from Incipio Group and the Southbank Centre to install a temporary pavilion on the roof of the building.
Royal Festival Hall was built in 1951 and is the only building left standing from the Festival of Britain. It was the first post-war building to receive the protective Grade I-listing and is regarded as an emblem for post-war optimism.
Permission for a Pergola on the River that offers visitors rooftop views of the River Thames will be considered by Lambeth Council, the Guardian revealed.
The arts centre told the newspaper it cannot introduce a plan for rooftop performances or make investments into the hall without revenue from a commercial space, which would operate for three years.
The space would be open to drinkers and diners between 11am and 1am on Sunday to Wednesday, and from 11am to 2am on Thursday to Saturday, with a maximum capacity of 200 people.
Opponents disagree with covering the distinct blue lettering and curved roof for such a long time, with the Twentieth Century Society conservation group condemning the plan also.
A spokesperson for Historic England said proposals would harm the character of the venue and recommended the application be rejected.
The group said: “The Royal Festival Hall is one of the most important and best-loved post war buildings in the country.
“We welcome the Southbank Centre’s intention to open up the riverside roof as a performance space by installing a permanent lift and stairs, but have serious concerns about the designs for the proposed temporary rooftop pavilion.
“We have objected to what would be a major insensitive intervention to a landmark building. The pavilion would largely obscure the iconic ‘Royal Festival Hall’ signage and much of the curved roof for three years.”
The group added: “We are sympathetic to the Southbank Centre’s needs to fund its important cultural work but we are not convinced that alternative locations on the Southbank Centre site have been fully explored, alongside other means of income generation.
“We consider that the harm to the significance of the Royal Festival Hall is not justified and recommend that the local authority refuses the plans or that they are withdrawn.”
The group said it would be pleased to discuss other methods of fulfilling the needs of the centre, “without harming the character of this remarkable building”.
Incipio, known for its ‘Instagrammable’ pop-up brands, will open a permanent Pergola venue on the roof of Olympia London, following the success of a venue in Paddington station. Pergola Olympia is part of the exhibition centre’s re-imagining and will open at the end of this month.
Incipio Group has been contacted for comment but had not responded by the time of publication.