“Whilst we hold hope that baby is still alive, we agree that the risk to the baby is extremely high,” police said.
Police officers are urgently searching the Roedale Valley Allotments in Brighton, and surrounding woodland near Golf Drive, a road up from the allotment.
Sky correspondent Sabah Choudhry reported that two police forces were searching the area. Special officers are also hunting for the baby, which is believed to be around two months old.
Officers have appealed for members of the public to remain “vigilant” in areas of open land or check any outbuildings they may have on their property.
More from UK
Detective Superintendent Lewis Basford told Sky News: “We are currently looking into all available areas that the baby could be in the open land.
“That will include the search of outbuildings and anywhere the baby could have been left by the couple to seek shelter away from the climate and extreme temperatures.
“The search will include every facility in this location, going out into the open land to try and locate the baby.”
DSI Basford said no further information on where the baby may be located has been gained from questioning the couple, while they remain in custody.
“I have always remained hopeful that the child is alive, and the investigation we have led is all about the safe recovery and location of the baby,” he added.
Speaking to The Independent, Marten’s father, Napier Marten, said it was “an immense relief” to know the couple have been found, but it would have been “far better if they had handed themselves in earlier”.
He added that it was “very alarming news her baby has yet to be found”.
Marten, 35, and Gordon, 48, were reported missing after their car broke down near junction four of the M61, near Bolton, on Thursday 5 January.
Over the following days, there were sightings of them in a number of places, including Liverpool, Essex, south London and East Sussex.
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.
In the days after they went missing, DSI Basford said the baby was “at the heart of the investigation” and that concerns were rising “day by day” for the child’s wellbeing.
Shereen Nimmo, director of midwifery for Barts Health NHS Trust, also issued an appeal urging the couple to bring the baby in for checks.
Marten, who is from a wealthy family, has lived an isolated life with Gordon since they met in 2016.