The remains of a baby have been found by police searching for the missing child of Constance Marten and Mark Gordon.
The discovery in a wooded area came as hundreds of officers and volunteers scoured a 90 square kilometer area near Brighton.
The couple disappeared in early January and were arrested earlier this week.
They are in custody on suspicion of grossly negligent murder.
Det Supt Lewis Basford of the Metropolitan Police, the force 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 I and many officers who have been part of this search had hoped would not happen,” he said.
“I recognize the impact this news will have on many people who have followed this story closely and can assure them that we will do everything we can to establish what has happened.”
He said a post-mortem examination will now be held to help investigators as they try to establish what happened. Previously, the police were given an extra 36 hours to question the couple.
Teams continue to comb the area by torchlight for evidence, the final stages of a 55-day search that has ended in tragedy.
A missing persons investigation into Marten and Gordon – a convicted rapist and registered sex offender – began when their car was found near Bolton on January 5.
It sparked a national search and sightings were reported in Liverpool, Essex and London, mostly limited to short bursts of CCTV footage.
Police believe they were sleeping outdoors in a tent and living off the grid, leading to fears for the child’s health in winter temperatures.
The last confirmed sighting of the pair before their arrest was in Newhaven on 8 January. Their baby was with them and alive at the time, police said earlier.
They were taken into custody at a convenience store on Monday night after a member of the public recognized them from media calls and called the police.
Officers were on the scene within six minutes and every member of the police force available in the region was called in to begin the massive search operation.
Marten and Gordon were first arrested on suspicion of neglect, and then arrested the next day. For grossly negligent homicide.
Hundreds of officers and volunteer rescue workers were seen scouring the vast area inch by inch for signs of life or clues as to the baby’s whereabouts.
It continued through the night and into Wednesday, with sniffer dogs, helicopters, drones and thermal cameras in use.
The fingertip search, carried out in near-freezing temperatures, extended east of Brighton to Newhaven, north of the city center to a nature reserve and on to the South Downs.
Witnesses told the BBC they had seen teams searching through thick brambles, allotment gardens and rubbish bins, while people living in the area were asked to look out for anything unusual.
On Tuesday, police said they still had some hope the baby might be alive, but said “this may not end the way we want it to.”
As late as Wednesday afternoon, people in the area were asked to “be alert” and look out for anything unusual when outdoors.
After confirming the discovery of a body, Det Supt Basford urged the public not to speculate on the matter.