Met Police Detective Superintendent Lewis Basford said the couple had not given them information about the child’s whereabouts or even disclosed its gender.
“We now have to consider the possibility that the baby has come to harm,” he told reporters.
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0:27
Missing couple moments before arrest
He said he still hoped the baby could be found alive but that the cold weather was an obvious risk to its safety.
“Clearly the risk is getting higher… this may not end in the way we would like, but we need to remain hopeful,” he said.
He added they were still open-minded about the possibility the child may have been given to someone else to look after.
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11:55
Police search for missing baby
Chief Superintendent James Collis, from Sussex Police, said the search area was “vast” at about 91 square miles and centred on land between Brighton and Newhaven – about seven miles further down the coast.
He also appealed for people to contact the force with any information or potential sightings over the last few weeks.
Marten’s father, Napier Marten, told The Independent it was “an immense relief” to know the couple had been located but “very alarming news her baby has yet to be found”.