A “diligent” member of the public, who had seen media reports about the couple, phoned 999 after witnessing them withdraw cash from a convenience store in Hollingbury Place shortly before 9.30pm.

Officers arrived six minutes later, with Marten and Gordon being detained on nearby Stanmer Villas.

Since the arrest, more than 200 officers have begun searching an area of seven miles by 13 miles for the baby, from where the couple were last sighted to where they were arrested – including allotments and woodland.

Detective Superintendent Lewis Basford said police still hoped to find the child safe and well, with a helicopter, sniffer dogs, thermal imaging cameras and drones all being used in the search.

However, confirming Marten and Gordon had been re-arrested on suspicion of gross negligence manslaughter, Mr Basford said police had to consider the possibility the child had “come to harm”.

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

Mr Basford said: “We have had a significant period of time in custody facility with both Constance and Mark.

“At this time we have not furthered that information, which has now obviously led to the position where we feel that the risk is getting so great that we now have to consider the possibility that the baby has come to harm.

“I can now confirm that they have been further arrested on suspicion of gross negligence manslaughter and that they remain in custody at police stations in Sussex.

“The baby was not with them and we have not found the baby as yet.”

The lengthy police search for Marten, 35, and Gordon, 48, began on January 5 when their car was found on fire abandoned next to the M61 in Bolton.

They used taxis to first travel to Liverpool, then Harwich in Essex, and on to east London, before arriving in Sussex on January 8.

The couple avoided detection 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.

Mr Basford said officers are looking into footage shared online of Gordon seen with a stick before he was arrested – adding that there was sufficient intelligence to suggest the pair had spent most of their time in outdoor open spaces while avoiding police.

The officer conceded the cold weather meant they had to be open to the investigation not “ending in the way we would like”.

The infant was born in early January and has had no medical attention since then, with its parents sleeping rough in freezing temperatures much of the time.

Earlier on Tuesday, Mr Basford said police were yet to locate items bought by Marten and Gordon at Argos on January 7 – adding: “We would still say we are actively looking for those.”

Marten’s father said it was a huge relief that his daughter had been found but added it was “very alarming” that her baby remains missing.

Napier Marten told the Independent he loves his daughter but it would have been “far better” if she and Gordon had handed themselves in earlier.

Marten, who is from a wealthy aristocratic family, was a promising drama student when she met Gordon in 2016.

Since then the couple have led an isolated life and, in September, as Marten’s pregnancy progressed, began moving around rental flats.

Gordon served 20 years in prison in the US for rape and battery committed when he was 14.

It is unknown if their baby was full-term or has any health issues.

In an appeal a week ago, Shereen Nimmo, director of midwifery for Barts Health NHS Trust, urged the couple to bring the baby in for checks.

She said: “You’re putting your baby at risk by not accessing medical care, so it’s really important that you come and see a midwife, doctor or another healthcare professional as soon as possible.

“Please do the right thing for your baby and go to your nearest healthcare facility so my caring colleagues can take care of you and your baby. All we want to do is help you.”