The Ministry of Justice said Jane Andrews was found missing during a Sunday evening roll call at the East Sutton Park open resettlement prison, a mansion about 45 miles southeast of central London.
Andrews was jailed for life in May 2002 after a jury found her guilty of killing her millionaire partner Thomas Cressman with a cricket bat and a kitchen knife, apparently in revenge for his refusal to marry her.
Cressman was found dead in bed Sept. 17, 2000 at the west London house the couple shared. Police found Andrews in her car three days later, after she had taken an overdose of painkillers.
Andrews claimed the death was an accident and that she acted in self-defense when Cressman tried to rape her.
An appeal of her conviction was turned down in 2003.
Andrews was a travel companion and accounts manager for nine years for the duchess, also known as Sarah Ferguson. She left the duchess' service three years before the murder.