The giant bust was conceived by the late Oscar Nemon, an acclaimed sculptor whose subjects included Sir Winston and the Queen. It has been revived by his son, Falcon Nemon Stuart, when he found details of the project in his father's papers.
Mr Nemon Stuart is hoping to win lottery funding from the Millennium Commission toward the pounds 1m to pounds 1.5m cost of the head, which would house a cafe in its plinth.
But Lady Soames told the Independent that neither she nor her nephew, Winston Churchill MP, had been consulted: "I've always admired Oscar Nemon's heads of my father, but I really dislike the idea of this monstrous big head. It reminds me of a Stalinist monument." She added that she had contacted Mr Churchill, who is abroad, and he had agreed with her.
"Oscar Nemon was a very good friend. But I can't think of a site where this giant head wouldn't be totally out of proportion," Lady Soames said.
"I shall do everything in my feeble power to stop it."
The opposition from Lady Soames will fuel the debate about the bust. But Mr Nemon Stuart remains convinced of what he sees as a "grand gesture". "Everybody I've spoken to thinks it's a great idea. That's the nice thing about it."
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