The risk to the missing baby of aristocrat Constance Marten and her partner Mark Gordon is "extremely high", police said today.
A huge search is underway for the two-month-old, involving more than 200 officers, police drones, dogs and a helicopter.
The infant has not received any medical attention since birth in early January.
Officers have been unable to gain any information on the baby's whereabouts from questioning Marten and Gordon, who remain in police custody.
It is unknown if the newborn, whose parents have been sleeping rough in freezing temperatures, was full-term or has any health issues.
Nicola Bulley's children 'cried their eyes out' after being told 'mummy's lost'In an off-camera briefing this afternoon, a Met Police spokesperson said: "Whilst we hold hope that the baby is still alive, we agree that the risk to the baby is extremely high.
"More than 200 officers have been engaged in the search and officers arrived within 6 minutes after the 999 call yesterday that led to the arrests of Constance Marten and Mark Gordon."
The police search for Miss Marten, 35, and Mr Gordon, 48, began on January 5 when their car was found on fire abandoned next to the M61 in Bolton.
They used taxis to first travel to Liverpool, then Harwich in Essex and onto east London, before arriving in Sussex on January 8.
The couple avoided detection by only making payments in cash, hiding their faces when on CCTV and often moving around at night or in the early hours of the morning.
They were finally arrested at about 9.30pm on Monday on suspicion of child neglect by officers from Sussex Police.
The couple were held in Stanmer Villas, Brighton, after a tip-off from a member of the public, but the baby was not with them.
The senior investigating officer, Detective Superintendent Lewis Basford, told BBC Breakfast that officers from the Metropolitan and Sussex police forces are searching a huge area of land to try to find the infant.
He 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 arrest location was close to open land and the couple were moving towards that land. And that's why we are focusing heavily on where we are right now.
Mum appeared 'completely normal' moments before vanishing while walking dog"We know that they travel, we know they travel long distances.
"My plea to the members of the public here in Brighton and crossing over towards Newhaven is please be vigilant in the open land where you are, in the outbuildings that you may have on your own property, and assist us in that way by being vigilant."
Marten, who is from a wealthy aristocratic family, 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 Marten's pregnancy progressed, began moving around rental flats.
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.
"Babies need a safe, warm environment and all new mums are given information about safe sleeping for their baby to prevent sudden infant death syndrome.
"Please do the right thing for your baby and go to your nearest healthcare facility so my caring colleagues can take care of you and your baby. All we want to do is help you."
At the time, Mr Basford said about 50 officers were working on the case at any one time and more than 630 hours of CCTV had been viewed.
He said there had been more than 350 calls to police from the public with information following previous appeals.
Anyone who may be able to assist in the search for the couple's baby is urged to contact 999.