Image caption Michael Arnett was said to have "experience" of child abduction

A Briton faces up to 27 years in a US jail for plotting to kidnap, rape, kill and eat a child, authorities say.

Agents found the basement of Geoffrey Portway's Massachusetts home equipped with a steel cage and a child-sized home-made coffin, in a raid last year.

The 40-year-old had spent months discussing the kidnap and murder online with an American, Michael Arnett.

Portway pleaded guilty to soliciting the kidnap of a child and distribution and possession of child pornography.

His admission on Monday was part of a plea agreement, the US Attorney's Office said. Under the terms he must serve between 216 and 327 months in jail before he is deported to the UK.

Real children

Detectives said Portway used programmes such as Skype to communicate with Michael Arnett over several months, asking him for help with the kidnap and setting out his preferred age range.

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

According to US court documents, Portway had solicited several people for help in obtaining a child, including Arnett.

The dungeon was described in detail by Portway in recovered chats as a place he intended to use to keep kidnapped children US Attorney's Office

Police believe he began making inquiries from at least 2010 and was informed that Arnett had previously helped others with such requests and had "experience" with the abduction and sexual abuse of children.

During their web conversations, Portway boasted of his intention to lock children in his basement, referencing scalpels, butchering kits, freezers and castration tools.

Detectives also found material showing that the men had shared child pornography and images of injured, mutilated and dead children.

During last year's raid, federal agents found a locked door in the basement, which led to a second door that opened into a dungeon lined with sound-deadening material containing a small makeshift coffin with external locking devices.

Nearby were a steel cage with multiple locks, and a steel table top with steel rings at six points, which police believe were intended to restrain victims.

Pornography collection

The dungeon was further kitted out with a chair, a television, and what appeared to be cable access to the internet, officers said.

Outside the room detectives found a chest freezer and an upright freezer, along with some disposable scalpels, butchering kits, and castration tools.

Various computers and digital devices containing Portway's child pornography collection were seized during the raid.

A forensic examination of the material revealed more than 4,500 pieces of child pornography traded with others online - including images and videos which appeared to depict dead children and cannibalism.

Authorities believe Portway was part of a worldwide network of offenders and, in many cases, distributed the material based on the preferences of his contacts.

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."

The UK Foreign Office said it was aware of the arrest of a British national in Massachusetts in July 2012 and consular assistance had been offered.