Missing aristocrat Constance Marten and her partner Mark Gordon are in police custody after being arrested in Brighton.
Metropolitan Police said an urgent search operation is underway to find their baby, who has not had any medical attention since the birth in early January.
The pair were located by officers from Sussex Police in Stanmer Villas, Brighton on Monday night after a member of the public reported seeing them shortly before 9.30pm.
Ms Marten, 35, and Gordon, 48, have been travelling around the UK by taxi since their car was found burning on the M61 in Bolton, Greater Manchester, on 5 January.
Ms Marten, who is from a wealthy aristocratic family, was a promising drama student when she first met her partner in 2016.
Since then the couple have led an isolated life and in September, when Ms Marten was well into her pregnancy, began moving around rental flats.
Gordon served 20 years in prison in the US for rape and battery committed when he was 14.
It is unknown if their baby was full-term or has any health issues.
