A Holocaust memorial proposed for outside Parliament would have a "significant harmful impact" on the area, the Royal Parks have said.
The landmark is planned to be built at Victoria Tower Gardens on Millbank, alongside the River Thames.
Royal Parks, which looks after the space, said it could not support the plans as the Grade II listed park was a "highly sensitive location".
The application is currently being considered by Westminster City Council.
The proposed memorial features 23 large bronze fin structures and an underground learning centre.
It will be dedicated to the six million Jewish men, women and children and other victims murdered by the Nazis.
The project was announced in 2016 by then prime minister David Cameron, with the works being led by the UK Holocaust Memorial Foundation.
In a letter to the council, Royal Parks said while it "strongly supports" the principle of the project, it thought the current design would have "significant harmful impacts" on the "character and function" of the park.
"The structure will dominate the park and eclipse the existing listed memorials which are nationally important in their own right," the charity said.
Royal Parks added the number of visitors expected would create "queues and congestion" and "change the nature of what is currently a relaxed park".
It has been predicted that one million people would visit the memorial in its first year.
Architect Sir David Adjaye, who has led the design team, previously said "disrupting the pleasure of being in a park is key to the thinking" of the memorial.
However, he later added that architects were working to ensure that 90% of the original park was retained.
More than 10,000 people have signed an online petition calling on the government to reconsider the location.
A spokeswoman for the Ministry of Housing, Communities and Local Government said "no location in Britain is more suitable for the area for the memorial".
"The proposals have been developed with great sensitivity to the existing context and character of the gardens," she said.