The remains of a baby have been found by police searching for the missing child of Constance Marten and Mark Gordon.
The discovery in an area of woodland came as hundreds of officers and volunteers were scouring a 90 square mile area near Brighton.
The couple disappeared in early January and were arrested earlier this week.
They are being held on suspicion of gross negligence manslaughter.
Det Supt Lewis Basford from the Metropolitan Police, the force which is leading the investigation, said a baby’s body had been discovered close to where the couple were arrested on Monday.
“This is an outcome that myself and that many officers who have been part of this search had hoped would not happen,” he said.
“I recognise the impact this news will have on many people who have been following this story closely and can assure them that we will do everything we possibly can to establish what has happened.”
He said a post-mortem examination will now be held to help detectives as they seek to establish what happened. Earlier, police were granted an extra 36 hours to question the couple.
Teams are continuing to comb the area by torchlight looking for evidence, the final stages of a 55-day search which has ended in tragedy.
A missing persons investigation surrounding Ms Marten and Gordon – a convicted rapist and registered sex offender – began when their car was found alight near Bolton on 5 January.
It triggered a national search and there were reported sightings in Liverpool, Essex and London, mostly limited to brief snatches of CCTV footage.
Police believe they were sleeping outdoors in a tent and living off-grid, prompting fears for the health of the child in winter temperatures.
The last confirmed sighting of the couple prior to their arrest was in Newhaven on 8 January. Their baby was with them and alive at that time, police said.
They were detained at a convenience store on Monday night after a member of the public recognised them from media appeals and called the police.
Officers were on the scene within six minutes and every member of police staff available in the region was drafted in to begin the massive search operation.
Ms Marten and Gordon were initially arrested on suspicion of child neglect, then were further arrested the following day.for gross negligence manslaughter.
Hundreds of officers and rescue volunteers were seen probing the wide area inch by inch for signs of life or clues about the baby’s whereabouts.
It continued through the night and into Wednesday, with sniffer dogs, helicopters, drones and thermal cameras all in use.
The fingertip search conducted in near-freezing temperatures extended east of Brighton to Newhaven, north of the town centre into a nature reserve and beyond to the South Downs.
Witnesses told the BBC they had seen teams searching through thick bramble, allotments and bins, while people living in the area were told to look out for anything unusual.
On Tuesday, police said they still had some hope the baby could be alive but said that “this may not end in the way we would like”.
As late as Wednesday afternoon, people in the area were asked to “remain vigilant” and watch for anything unusual when outdoors.
After confirming the discovery of a body, Det Supt Basford urged the public not to speculate about the case.