A woman and her partner who went missing in the UK in January are in police custody after being arrested in Brighton.
Metropolitan Police said an urgent search operation is under way to find their baby, who has not had any medical attention since birth in early January.
The force said Constance Marten and Mark Gordon were located by officers from Sussex Police in Stanmer Villas, Brighton last night after a member of the public reported seeing them shortly before 9.30pm.
Ms Marten, 35, and Mr 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 comes from a wealthy aristocratic family.
Authorities previously believed the couple had been sleeping rough in a blue tent.
Today, police said the couple has been further arrested on suspicion of gross negligence manslaughter as their baby remains missing.
Detective Superintendent Lewis Basford said: "I can confirm that they were initially arrested on suspicion of child neglect.
"I can now confirm that they have been further arrested on suspicion of gross negligence manslaughter and that they remain in custody at police stations in Sussex.
"The baby was not with them, and we have not found the baby as yet."
Detective Superintendent Basford said officers from the Metropolitan and Sussex police forces are searching a huge area of land to try to find the infant.
The couple are known to have travelled long distances during the time they have been missing.
DS Basford said: "It is a vast area and at present we are looking in the local vicinity to where the couple were arrested last night, seeking a shelter or location for where they may have been holed up and hopefully where the baby is present."
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 8 January.
They spent time sleeping rough, often moving around at night and in the early hours of the morning to avoid detection.
Ms Marten was a promising drama student when she first met Gordon in 2016.
Since then the couple have led an isolated life, and in September, as Ms Marten's pregnancy progressed, began moving around rental flats.
Mr 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.
In an appeal a week ago, Shereen Nimmo, director of midwifery for Barts Health NHS Trust, urged the couple to bring the baby in for checks.
She said: "You're putting your baby at risk by not accessing medical care, so it's really important that you come and see a midwife, doctor or another healthcare professional as soon as possible."
Ms Marten's father has spoken of his "immense relief" after she was found.
Speaking to The Independent, Napier Marten said: "It is an immense relief to know my beloved daughter Constance has been found, tempered by the very alarming news her baby has yet to be found.
"For whatever reasons she and her partner went on the run, the consequences of their actions have increased many fold. It would have been far better if they had handed themselves in earlier."
He said he was "profoundly grateful" to the police for committing "great resources" to searching for his daughter.
"When the time comes, I am longing to see Constance to reassure her that, whatever the weather, I love her dearly and will support her as best I can through the difficult weeks and months here on in," he added.
