Police believe the baby of Constance Marten and Mark Gordon "may have been dead for several weeks".
Officers have also been unable to confirm the gender of the baby, Detective Superintendent Lewis Basford of the Metropolitan Police said in an update on Thursday afternoon.
Constance Marten, 35, and Mark Gordon, 48, were arrested on suspicion of child neglect on Monday after they were spotted in Brighton.
They were re-arrested the following day on suspicion of gross negligence manslaughter and the remains of a baby were found on Wednesday. The couple had reportedly been on the run since January 5 when their car was found abandoned and on fire next to the M61 in Bolton.
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In an update issued by the Met Police, De Supt Basford said: "My team has been working on this investigation for the past eight weeks and we have travelled across the country as part of our searches for this baby. We are all truly devastated by the outcome and we know this emotion is also being felt here in Brighton and across the country today.
"While there are still many unanswered questions 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. At this stage we have not yet been able to confirm the baby's gender and a post-mortem examination has not yet taken place.
"Despite this, based on the enquiries we've carried out so far, we believe, sadly, the baby had been dead for several weeks before they were found. It's 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. This is standard protocol for such circumstances.”
He added that Met officers will remain in the Brighton area over the coming days as inquiries continue.
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