Constance Marten and Mark Gordon arrested on suspicion of manslaughter

Constance Marten and Mark Gordon have been further arrested on suspicion of gross negligence manslaughter as police search for their missing baby.

A vast outdoor area in Brighton was searched through the night, and police say the infant could still be alive.

But with no trace yet, investigators fear the baby may have "come to harm".

The baby's parents were arrested on Monday - initially on suspicion of child neglect - after being missing for 53 days.

They were held after a member of the public saw them in a shop and called the police.

They remain in custody, but police said they had not provided any further information about the condition or whereabouts of the child.

The search for Ms Marten and Gordon - a convicted rapist and registered sex offender - began when their car was found ablaze by the side of a motorway on 5 January.

Police established Ms Marten had recently given birth, possibly in the back of the vehicle, without a midwife or any medical attention.

Speaking at a press conference on Tuesday, Detective Superintendent Lewis Basford, from the Metropolitan Police, said the risk to the baby - now thought to be about two months old - increased as time went on, especially amid cold weather.

He added that police must now "be open to the fact this may not end in the way we would like".

More than 200 officers are searching a 91-square-mile site in Sussex.

Sniffer dogs, drones, a police helicopter and thermal imaging cameras are being used in the search.

Police have asked members of the public between Brighton and Newhaven, in East Sussex - where the previous last known sighting of the couple was made on 8 January - to report any potential sightings, or information about where they may have been sleeping.

Residents near Stanmer Villas, where the couple were arrested, have also been asked to check sheds and outbuildings.

Investigators believe Ms Marten and Gordon have been living "in open land and open areas".

Allotments and a golf course next to a nature reserve, near to where they were arrested, were intensively searched throughout Tuesday.

Police said every available officer and member of staff had been drafted in to assist with the massive search.

• CCTV shows how search for missing couple unfolded

Det Supt Basford told journalists that little information had been gathered during police interviews with the couple and that police did not yet know the sex of their baby.

Constance Marten and Mark Gordon disappeared with a newborn baby more than seven weeks ago

Police have not ruled out that someone could be harbouring the baby, though they said this was unlikely.

After the car fire in January, police said the family safely left the scene and travelled to Liverpool, Essex, London and East Sussex in quick succession.

They appear to have taken steps to avoid detection, including covering their faces when in public, travelling at night and using cash to purchase supplies.

Police previously said Ms Marten's inherited wealth may have allowed the couple to remain at large for an extended period.

Police believe the baby was alive at the time of the last previous sighting of the family in Newhaven, but little more is known about the family's movements in the weeks since.

At the end of January, police offered a £10,000 reward for information, appealing especially to anyone who might have helped the couple.

Aerial view of Crichel House, believed to be Ms Marten's childhood home

Their home is in Eltham, in south-east London, but they have been living nomadically since September last year, when Ms Marten first started to show signs of pregnancy.

After the pair were found on Monday, Ms Marten's estranged father Napier Marten told the Independent he felt "immense relief", though this was "tempered by the very alarming news [her] baby has yet to be found".

Ms Marten, 35, is from a privileged background, having lived in a stately home growing up and attended private school.

She became estranged from her family after meeting 48-year-old Gordon at drama school in 2016.