LONDON — A man whom British prosecutors called “a serial liar and a fraud” for making up claims of sexual abuse against high-profile public figures — fabrications the authorities said cost 2 million pounds to investigate — was sentenced to 18 years in prison on Friday.

The man, Carl Beech, 51, was found to have made up allegations against — among others — Ted Heath, a former prime minister, and Jimmy Savile, the disgraced television personality who sexually abused 72 victims, including children.

Mr. Beech claimed that he had been taken out of school, transported to sex parties and abused at more than 20 locations in London and southeastern England from the ages of 7 to 16, by a group of prominent public figures in a pedophile ring. Among his most explosive and serious allegations, reported at length in the British news media, was that he had witnessed the murders of three children.

Along the way, Mr. Beech built a public persona as an ally for victims of child sexual abuse. He wrote a blog, worked on his memoirs and spoke at a conference on living with the effects of abuse. Prosecutors said Mr. Beech received £22,000, about $27,000, in compensation.