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 birth in early January.
The force said 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.
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 January 5.
Authorities previously believed the couple had been sleeping rough in a blue tent, and had avoided being traced by the police by moving around frequently and keeping their faces covered in CCTV images.
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.
Miss Marten, who is from a wealthy aristocratic family, was a promising drama student when she first met Mr Gordon in 2016.
Since then the couple have led an isolated life, and in September, when Miss 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.
Anyone who may be able to assist in the search for the couple’s baby is urged to contact 999.
