Constance Marten and her partner Mark Gordon are arrested - but their baby is still missing

A mother and father who went missing with their newborn baby have been found and arrested, police say, although the baby is still missing.

Sussex Police said that Constance Marten and Mark Gordon were spotted in Brighton by a member of the public just before 9.30pm on Monday.

They were arrested on suspicion of child neglect and are in police custody.

Their baby is still missing, however, and an urgent search operation is under way in the area, police said.

Detective Superintendent Lewis Basford told Sky News: "We are currently looking into all available areas that the baby could be in the open land.

"That will include the search of outbuildings and anywhere the baby could have been left by the couple to seek shelter away from the climate and extreme temperatures.

"The search will include every facility in this location, going out into the open land to try and locate the baby."

DSI Basford said no further information on where the baby may be located has been gained from questioning the couple, while they remain in custody.

Watch: Constance Marten and her partner Mark Gordon are arrested - but their baby is still missing

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"I have always remained hopeful that the child is alive, and the investigation we have led is all about the safe recovery and location of the baby," he added.

Sky correspondent Sabah Choudhry reported there were between 10-15 officers searching the Roedale Valley Allotments in Brighton.

Police are appealing for members of the public to be "vigilant" in areas of open land, and any outbuildings they may have on their property.

Marten, 35, and Gordon, 48, were reported missing after their car broke down near junction four of the M61, near Bolton, on Thursday 5 January.

Over the following days, there were sightings of them in a number of places, including Liverpool, Essex, south London and East Sussex.

Authorities previously believed the couple had been sleeping rough in a blue tent, and had avoided being traced by the police by moving around frequently and keeping their faces covered in CCTV images.

In the days after they went missing, DSI Basford said the baby was "at the heart of the investigation" and that concerns were rising "day by day" for the child's wellbeing.

Ms Marten, who is from a wealthy family, has lived an isolated life with Mr Gordon since they met in 2016.