Two Labour MPs in the left-wing Campaign group, acting as his agents, paid a late-night visit to the crypt and, using a cordless electric drill, put up the plaque without permission.
Mr Benn's new plaque contains a long list of historical figures engaged in the struggle for democracy, some of whom may be obscure to anyone unfamiliar with Mr Benn's speeches.
The broom cupboard of the crypt, where many christenings have taken place over the years for the offspring of the privileged and aristocratic, may be the least likely location for a memorial to socialism. But it is where the suffragette martyr Emily Wilding Davison hid on the night of the 1911 census to claim equal rights with men as a resident of the Commons.
She died after throwing herself under the King's horse in the Derby in 1913. Mr Benn placed a plaque to her memory with a photograph of her in the cupboard saying: "By such means was democracy won for the people of Britain."
His new memorial says: "This plaque is dedicated with respect and gratitude and affection to the many hundreds of thousands of men and women who lived and worked on these islands and who devoted themselves to the advancement of freedom, civil liberties, social justice and democracy and campaigned for popular representation in Parliament."
The roll of honour lists: Wat Tyler, John Ball, William Tyndale, Thomas More, The Levellers, John Liburne, William Walwyn, the Diggers, Gerrard Winstanley, Tom Paine, Mary Wollstonecraft, Robert Owen, the Tolpuddle Martyrs, the Chartists, Keir Hardie, Annie Besant, Charles Bradlaugh, Robert Tressel, the Suffragettes, Constance Markiewicz . . . and many others whose names have never been recorded in our history." It adds: "Placed here by Tony Benn, 1995.''
One of the MPs said: "I suppose we will get our wrists slapped for this. But it was worth it."Reuse content