A British judge has sentenced a husband and wife who kept a man as a slave for almost a quarter of a century to six years each in prison.

Emmanuel Edet, a doctor, and his wife Antan Erdet, a nurse, moved from Nigeria to Britain in 1989. With them came 13-year-old Ofonime Sunday Inuk - whom they told officials was their son.

Prosecutors say he worked as an unpaid "houseboy" for 24 years, cooking, cleaning, gardening and looking after the couple's two sons.

Inuk eventually contacted a charity after the couple went to Nigeria for Christmas 2013 and they were arrested three months later.

Sentencing the Edets on Monday at London's Harrow Crown Court, Judge Graham Arran said their behavior robbed Inuk "of the opportunity of leading a normal life."