An application to install a bar on the roof of the Royal Festival Hall on London's South Bank has prompted "serious concerns".
The Pergola on the River, overlooking the River Thames, would be a temporary addition to the historic riverside building, which the applicants have said would enhance the arts and cultural offer as well as being a source of income and creating 30 new jobs.
A Historic England spokesperson said: "The Royal Festival Hall is one of the most important and best-loved post-war buildings in the country. It is the only building remaining from the 1951 Festival of Britain and is Grade I-listed for its outstanding architectural and historic interest.
"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. 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of the Royal Festival Hall is not justified and recommend that the local authority refuses the plans or that they are withdrawn. We would be pleased to continue discussions about other possible ways to fulfil the Southbank Centre's needs without harming the character of this remarkable building."