A British man has admitted plotting to rape, kill and eat children in a dungeon built into his home in Massachusetts.

Geoffrey Portway faces up to 27 years in a US prison before he’ll be deported back to Britain, a sentence negotiated under the terms of a plea agreement.

The 40-year-old spent months chatting with individuals online about a “mutual interest in abducting, raping, murdering, and eating children”, before going on to discuss specific plans to kidnap and murder a child in an underground, soundproof chamber.

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In the web chats, Portway boasted of his intention to lock children in the basement of his Massachusetts home, which was equipped with a steel cage, a child-sized home-made coffin, scalpels, butchering kits, freezers and castration tools.

A spokesman for the US Attorney's Office in the district of Massachusetts said: “This dungeon was described in detail by Portway in recovered chats as a place he intended to use to keep kidnapped children while he sexually abused them and as a place to eventually murder and cannibalise the children.”

In July last year, the FBI searched Portway’s home and discovered the dungeon, which had been described in detail in web chats. It was during those same chats that Portway stated his intention to kidnap, sexually abuse and eventually murder and eat children.

The conversations were also used to exchange child pornography and images of mutilated, injured or deceased children, as well as to solicit the help of Michael Arnett, a man Portway believed may be able to kidnap a child for him.

Police said records of the conversations revealed that the two men discussed real children - by name and photo - who Arnett claimed to know and have access to.