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 mile area near Brighton.
The couple disappeared in early January and were arrested earlier this week.
They are suspected of grossly negligent homicide.
Detective Superintendent Lewis Basford of the Metropolitan Police, the force leading the investigation, said the body of a baby was discovered near where the couple were arrested on Monday.
“This is a result that myself and many of the officers who participated in this research 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 happened.”
He said a post-mortem examination will now take place to help detectives seek to establish what happened. Previously, police had been granted an additional 36 hours to question the couple.
Crews continue to comb the area by torchlight for evidence, the final stages of a 55-day search that ended in tragedy.
An investigation into the missing people surrounding Ms Marten and Gordon – a convicted rapist and registered sex offender – began when their car was found burnt out near Bolton on January 5.
It sparked a nationwide search and sightings were reported in Liverpool, Essex and London, mostly limited to brief snippets of CCTV footage.
Police believe they slept outside in a tent and lived off the grid, raising fears for the child’s health in winter temperatures.
The last confirmed sighting of the pair before their arrest was in Newhaven on January 8. Their baby was with them and alive at the time, police said previously.
They were arrested at a convenience store on Monday evening after a member of the public recognized them from media calls and called the police.
Officers were on the scene within six minutes and all available police personnel in the area were mobilized to begin the massive search operation.
Ms Marten and Gordon were first arrested on suspicion of child neglect, then arrested the following day for gross negligence manslaughter.
Hundreds of rescue officers and volunteers were seen probing the vast area inch by inch for any signs of life or clues to the whereabouts of the baby.
It continued through the night and into Wednesday, with sniffer dogs, helicopters, drones and thermal imaging cameras all in use.
The fingertip search conducted in near-freezing temperatures stretched east from Brighton to Newhaven, north of the city center into a nature reserve and beyond to the South Downs.
Witnesses told the BBC they saw teams rummaging through thick brambles, plots and bins, while locals had to watch out for anything unusual.
On Tuesday, police said they still had some hope the baby was alive, but said ‘it may not end the way we would like’.
As recently as Wednesday afternoon, locals were urged to “stay alert” and watch for anything unusual outside.
After confirming the discovery of a body, Detective Superintendent Basford urged the public not to speculate about the case.