Police believe the baby of Constance Marten and Mark Gordon "may have been dead for some time".
Detective Superintendent Lewis Basford said the baby found in the Marten and Gordon investigation "may have been dead for some time" and the case has been referred to the police watchdog.
Speaking at a press conference on Thursday, the Metropolitan Police officer said: "At this stage we have not yet been able to confirm the baby's gender and post-mortem examination has not yet taken place.
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"Despite this, based on our inquiries we've carried out so far, we believe sadly the baby may have been dead for some time before they were found. It is too early for us to provide a more specific date.
"Because we believe the death occurred during the course of a missing person investigation we have made a mandatory referral to the Independent Office for Police Conduct (IOPC). This is standard protocol for such circumstances."
DS Basford said officers investigating the case were "devastated" by its outcome as he confirmed Marten and Gordon remain in custody. The warrant for further detention granted on Wednesday lasts until Thursday evening, he said.
He added: "We are truly devastated by the outcome and we know this emotion is being felt here in Brighton and across the country today. We know there are still many unanswered questions, and it is important that we give the investigation team the time and space they need to establish more details about the circumstances of this tragic death."
He added that Met officers will remain in the Brighton area over the coming days as inquiries continue.
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