A US Supreme Court has unanimously upheld Kennedy cousin Michael Skakel’s murder conviction, more than 30 years after a killing that sparked decades of intrigue.

Kennedy nephew Michael Skakel was jailed for 20 years to life today for murdering the girl next door in 1975 when they were 15-year-old neighbours.

The murder case against Kennedy cousin Michael Skakel will proceed, a US judge ruled yesterday, despite credibility questions about a key witness who admitted to recent heroin use.