Police have requested more time to question Constance Marten and Mark Gordon as a search for their baby intensifies.
The couple, who had been missing since January, were arrested on suspicion of child neglect by police on Monday but their newborn was not with them at the time.
Officers launched an urgent search to find the baby, who is understood to have gone without medical attention since birth.
Ms Marten and Mr Gordon, who have not provided any information relating to the baby’s location, were further arrested on suspicion of gross negligence manslaughter on Tuesday, police said.
Detective Superintendent Lewis Basford, Metropolitan Police, told journalists on Wednesday: “I can confirm that Constance Marten and Mark Gordon remain in police custody here in Sussex. An application for an extension of their detention for a further 36 hours has been sought from Brighton Magistrates Court.”
Mr Basford added: “The police searches for the baby continue, and we must consider that the baby has come to serious harm.”
The “hugely difficult and painstaking search operation” is covering a vast area of some 90 square miles.
Police have deployed helicopters, sniffer dogs and thermal imaging equipment in their attempts to locate the baby, which has been stepped up as concerns over the infant’s welfare grow.
“Detectives from the Met’s Specialist Crime Command are now leading inquiries to locate the baby because of the resources, technology and expertise available to them in this very complex investigation,” Mr Basford added.
Officers issued a further appeal to the public to “remain vigilant” and come forward with relevant information.
“If you’re out walking in these areas and you discover something you think that we should know about, please don’t hesitate contact us no matter how insignificant it may seem to you,” Mr Basford said.
Hundreds of officers from the Met and Sussex Police have been searching for Ms Marten and Mr Gordon’s baby “against the clock” due to the risk posed by the climate and weather conditions – with overnight temperatures close to freezing in Brighton.
This story is being updated