Constance Marten and Mark Gordon have been charged after the remains of a baby were found in a wooded area.
The Crown Prosecution Service said the aristocrat and her partner were also accused of concealing the birth of a child and perverting the course of justice. An infant's remains were found on Wednesday after a major two-day search operation in Sussex.
Marten, 35, and Gordon, 48, were arrested on Stanmer Villas in Brighton on Monday after avoiding the attention of police for several weeks. On Thursday, the Metropolitan Police said it is "too early" to provide a specific date of death, adding that the infant had been dead for "several weeks".
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They are due to appear at Brighton Magistrates' Court on Friday.
Barry Hughes, chief crown prosecutor for CPS London North, said: "The CPS has authorised the Metropolitan Police to charge Constance Marten and Mark Gordon with gross negligence manslaughter. Constance Marten, aged 35, and Mark Gordon, aged 48, have also been charged with concealing the birth of a child and perverting the course of justice.
"These charges arise from their arrest on Monday as a result of a lengthy police investigation to establish their whereabouts and that of their baby. The Crown Prosecution Service reminds all concerned that criminal proceedings against Constance Marten and Mark Gordon are active and that they have the right to a fair trial."
The Met said a post-mortem examination would take place on Friday. Marten and Mr Gordon were travelling around the UK by taxi since their car was found burning on the M61 in Bolton on January 5. The couple travelled from Bolton to Liverpool, then to Harwich in Essex, then to east London and then to Newhaven in Sussex, where they were seen near the ferry port on January 8.
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