James Sheen, 39, from Wellingborough in Northamptonshire, pleaded guilty to burglary, transferring criminal property and conspiracy to do the same at Oxford Crown Court on Tuesday (March 2).
The 18-carat toilet was stolen in September 2019 while it was featured in an art exhibition.
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Sheen appeared via video link from HMP Five Wells. He is already serving a 17-year sentence for a string of thefts, including from ATMs.
Sheen also stole £400,000 worth of tractors and high-value trophies from the National Horse Racing Museum in Newmarket.
Three other men have pleaded not guilty to charges related to the theft. Michael Jones, 38, of Clarks Row, Oxford, is accused of burglary.
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Frederick Does (formerly Sines), 35, of North Street, Winfield, Windsor, and Bora Guccuk, 40, of west London, are both accused of conspiring to transfer criminal property.
They are due to go on trial on February 24 next year, a proceeding estimated to take four to six weeks.
The art installation, entitled ‘America’, was available for visitors to look at and sit on. It is the work of artist Maurizio Cattelan and was plumbed in the palace, near the room where Winston Churchill was born.