Police searching for Constance Marten and Mark Gordon's two-month-old baby said the child's parents have now been arrested on suspicion of gross negligence manslaughter.
Hope is fading fast for the child as a huge search in Sussex continues tonight.
Speaking in Brighton, detective superintendent Lewis Basford said that the risk to the couple’s missing baby was getting “so great” that officers “have to consider the baby has come to harm”.
The pair, who had been missing for 53 days, were avoiding police since the infant was born in early January, moving around the country, paying for everything in cash and covering their faces when on CCTV.
They were arrested at about 9.30pm last night on suspicion of child neglect by officers with Sussex Police. They were re-arrested again this afternoon.
A huge 200-officer strong search is underway for the child, involving police drones, dogs, and a helicopter.
The infant has not received any medical attention since birth in early January.
Officers were unable to gain any information on the baby's whereabouts from questioning Marten and Gordon, who remained in police custody.
The couple continued to refuse to co-operate with police and refused to even tell them the sex of the child.
The force feared the baby had been sleeping rough with its parents.
Detective Superintendent Lewis Basford told reporters: "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. They remain in custody at police stations in Sussex.
"The baby was not with them, and we have not found the baby yet."
He added: "We still have hope that the baby can be found," he added but said that, as time progresses, the risk gets greater.
“We must retain that hope that the baby is found safe and well.
"However as time progresses, with the weather closing in as it is and the impact the cold would have on a baby, the risk is getting higher and this may not end in the way that we like,” he said.
Police earlier indicated that Ms Marten and Mr Gordon could have entrusted a third party with their missing baby.
In update on the search this afternoon, Det Supt Basford suggested they might have offered their “cash reserves” to a non-law abiding individual in exchange for their baby's protection.
He said: “Whilst we’re still in the position, we hold hope the baby is safe and we agree that the risk is extremely high."
Following the arrests, Constance's father Napier Marten said he was "very alarmed" that the baby remained missing.
He said that he loves his daughter dearly, but it would have been "far better" if she and Gordon handed themselves in earlier.
Mr Napier told The Independent: 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 thanked the member of the public who tipped off police about the couple's whereabouts, as well as Metropolitan police officers for their "dogged investigation".
Anyone who may be able to assist in the search for the couple's baby is urged to contact 999.
