Detective Superintendent Lewis Basford called on the public to report suspicious behaviour and items as an urgent search continues for the baby of aristocrat Constance Marten and her partner Mark Gordon.

He said officers had to consider whether the baby had come to "serious harm", calling it a "very complex investigation" involving a "hugely painstaking search operation".

Mr Basford said Marten and Gordon remained in police custody - and an application to extend their detention for a further 36 hours had been made to the courts.

"We're using every resource we have at our disposal to find the baby," he said.

Mr Basford urged the public to remain vigilant and continue providing information to the investigation team.

The couple were arrested in Brighton on Monday on suspicion of child neglect after a member of the public spotted them withdrawing cash at a shop in Hollingbury Place at about 9.30pm.

They were further arrested on suspicion of gross negligence manslaughter on Tuesday.

Since then, more than 200 police officers have been looking for the child, with the search focussed on an area including allotments and woodland in the city.

Det Supt Basford said officers had received no information about the welfare or location of the child since the pair were detained.

Marten, 35, and Gordon, 48, were reported missing on 5 January after their car caught fire on the M61, near Bolton.

It is believed Marten had given birth just a few days before and had not been assessed by medical professionals.

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 only making payments in cash, hiding their faces on CCTV and often moving around at night or in the early hours of the morning.

Sheds broken into in police search

Specialist teams have been scouring parkland around a mile from where the aristocrat and her partner were arrested in Brighton, with officers concentrating on an allotment and woodland in the city.

A helicopter, sniffer dogs, thermal cameras and drones joined the search this week, which widened to include the area towards Newhaven, with Met and Sussex Police officers working through the night into today.

Officers have searched every shed at the Roedale Valley Allotments and will remain on site for some time, users have been told.

Detectives had forced entry to outbuildings, said an entry on the allotments society Facebook page.

Read more:
From Liverpool to Brighton - mapping the sightings
CCTV breakthrough suggests couple 'sleeping in tent'

Groups of officers from the Metropolitan Police and London Search and Rescue searched under bushes and around trees for Marten's baby as they conducted sweeps of Moulsecoomb Wild Park.

Some officers were dressed in police uniform, while others were in red coats, black clothing or orange hi-vis, using sticks to wade through the undergrowth.

By midday, members of the team had been on shift for 14 hours, and had walked 13km in search of the baby.