Fugitive aristocrat Constance Marten’s baby could have died from the cold or co-sleeping, a pathologist has said.

Marten, 36, and her partner Mark Gordon, 49, are accused of the death of their newborn daughter Victoria while they were on the run from authorities last year.

They allegedly lived in a tent off-grid on the South Downs in a bid to keep Victoria, after their four other children were taken into care.

Days after they were arrested last February, Victoria was found dead inside a Lidl bag in an allotment shed in Brighton, East Sussex.

Dr Nat Cary told the Old Bailey on Thursday that baby Victoria was wearing just a nappy and had signs of “significant decomposition” when he examined her.

He told jurors that wintry temperatures slowed down the rate of decomposition but the condition of the body meant Victoria had not died recently.

Dr Cary said the post-mortem examination found the cause of death was “unascertained”.

He said there were no signs of head injury, natural disease, congenital abnormalities, or any injuries consistent with an assault or constraint of the child.

Asked to consider possible reasons for Victoria’s death in relation to the facts of the case, Dr Cary said: “A number of causes have been ruled out.

“Really when you start to take into account the circumstances, which is really a matter for the jury, hypothermia, being out in a cold environment, is a very important cause of death.

“The other would be co-sleeping – sharing facilities with another person where overheating may apply or rebreathing, breathing in expired air.

“It is fair to say none of that is provable. You cannot do a blood test.”

Another possibility was obstruction of breathing, he said: “At this stage it is very easy to impair breathing without any obvious signs in post-mortem examination.”

Cross-examining for Marten, Francis FitzGibbon KC raised a circumstance in which a child died when a mother fell asleep while feeding.

He said: “I’m not talking here about a policy, a plan of putting a baby down in a bed next to mother.

“I’m talking about mother feeding baby, drops off, exhausted, wakes up and baby has died.

“That’s a phenomenon that can occur anywhere with a tired breast-feeding mother.”

Dr Cary agreed that possibility could not be excluded.

Mr FitzGibbon said: “You cannot determine when the baby died.”

The pathologist replied: “I cannot.”

John Femi-Ola KC, for Gordon, highlighted there were no signs of violence on Victoria’s body. The witness agreed.

Andreas Marnerides, a consultant in paediatric pathology, told jurors he could not say exactly how old Victoria was from her foot length and other parameters.

She could have been two, four or six weeks old, he said.

Mr Marnerides added: “I cannot use a specific set of measurements to say, specially with this degree of decomposition, the baby must have been two weeks.”

Jurors were shown a replica of the red Lidl bag with sunflower, lemon and pepper motif in which Victoria’s body was found in.

The court was told the defendants’ sleeping bags had a rating designed for “late spring or early autumn”.

A series of photographs was shown to the jury of Marten following her arrest.

She was wearing leggings with holes in, and stuffing used as insulation poking out of her top and inside her shoes.

Previously, the court has heard that Marten told police Victoria had died after she fell asleep in a tent while holding her under her jacket.

According to facts agreed between the parties, she had previously been warned by social workers of the risks of falling asleep with the baby on her and that a tent would be “wholly inappropriate for a baby to live in”.

The defendants, of no fixed address, deny manslaughter by gross negligence, perverting the course of justice, concealing the birth of a child, child cruelty and causing or allowing the death of a child.