Constance Marten and her boyfriend Mark Gordon broke cover after running out of cash and food, The Telegraph can reveal as the frantic search for their missing baby continued.

The 35-year-old woman from an aristocratic family and Gordon, 48, had used a slush fund of thousands of pounds to live “off grid” to evade detection.

But, it has now emerged that they were forced to take increasingly desperate measures to get provisions after their stash of hard cash ran out.

The couple are understood to have survived on handouts from food banks for nearly a week before their arrest in Brighton on Monday.

However, the pair triggered a police alert that night when they used a cash card to withdraw money from a monitored bank account from an ATM at Mulberrys convenience store in the city.  The alert is believed to have come just moments before a member of the public spotted them buying supplies at the same shop and dialled 999.

The couple’s bank was being monitored by police in case funds were accessed and a location for their whereabouts was then established.

A Met spokesman said: “Simultaneous to the call from the member of the public, the investigation team was alerted to the use of a bank card. The pair were arrested minutes later.”

Nothing was purchased for their baby

However, staff at both the food bank and shop where they bought supplies have revealed that nothing was purchased for their baby.

They had obtained milk, pasta and rice from volunteers at the Brighton Food Bank in the Calvary Church Building last Wednesday, before apparently returning to their makeshift shelter on the edge of the South Downs.

But the couple did not pick up anything obviously intended for a baby, such as nappies or powdered milk, which were available at the food bank.

The couple were seen using a cash machine outside the Mulberrys convenience store on Hollingbury Place on Monday.

It was not clear whether the cash withdrawal in Brighton outside the convenience store was the first time Miss Marten and Gordon had used their cash card or activated a police alert that the bank account had been accessed.

Inside the shop they bought “crisps, soft drinks and other provisions” a member of staff said, although these did not appear to include anything for a baby.

The shop is a short walk from where they were arrested at around 9.30pm after a member of the public who spotted them and recognised from media coverage alerted the police.

They were given tinned food and bread

A female food bank worker said: “They were given tinned food, bread, pasta, milk and rice.

“They were not registered with us, which is how we usually operate.

“The baby was not with them. We have supplies here but they didn’t ask for anything for a baby. They came asking for help for themselves.”

The food bank worker, who only recognised the couple from photographs after they were arrested, added: “She did the talking. She asked for help. She was wearing a lot of layers and looking back she looked like she’d been outside for a while.

“She did the talking. We didn’t take any details, we just gave them food because we thought they needed help.”

On Tuesday, police said that while they held out hope they will find the baby alive, they admitted they fear the baby “may have come to some harm”.

Hundreds of officers scouring area

Hundreds of officers are scouring huge swathes of the Sussex Downs and Brighton.

Officers from London Search and Rescue could be seen searching Moulsecoomb Wild Park – around a mile from where the couple were arrested - on Wednesday.

The officers, some wearing jackets indicating they were dog handlers, searched underneath sticks and logs close to where the area meets Hollingbury Golf Course.

A uniformed officer stood guard at the entrance to the park, with dog walkers saying they saw police tape cordoning off parts of the woodland. Searches of nearby allotments continued.

The couple have refused to reveal the whereabouts of their baby or even tell police whether it is a boy or a girl. They were initially arrested on suspicion of child neglect but were then arrested on Tuesday afternoon on suspicion of gross negligence manslaughter.

The pair had disappeared on January 5 after their abandoned car was discovered on fire on the M61 near Bolton. They evaded detection for nearly two months after travelling to Liverpool, Colchester, London and Sussex.

With temperatures on the South Downs plunging to below freezing over the past few nights many of those who live in the area could not help but fear the worst.

'Can’t bear to think of that poor baby'

Danielle Mansell, 32, herself a mother of three, said: “I just can’t bear to think of that poor baby still being up there, if it’s even still alive, the poor thing.

“My youngest is just three and the thought of a little child being out there in this cold - it’s just too unbearable to think about.”

Greenhouses and sheds across the area have been inspected as the police teams scour the area for any sign of the baby, or the blue tent Ms Marten and her partner were reported to have been using to live rough over the past few weeks.

Police have searched every shed at the Roedale Valley Allotments near where Ms Marten and Mr Gordon were arrested and will remain on site for some time, users have been told.

An entry on the allotments society Facebook page said officers had forced entry to outbuildings as the hunt for the two-month-old baby continued.

One local resident reported a sighting of a tent on the Downs that may have been used by the couple in recent days.