An expert told jurors it was “exceedingly unlikely” fugitive mother Constance Marten’s newborn baby died from the cold.

Marten, 36, and her partner Mark Gordon, 49, were living off grid in a tent on the South Downs when their daughter Victoria died, the Old Bailey has heard.

In her evidence, Marten has said the child died when she fell asleep while holding her under her jacket in the tent last January 9.

Victoria’s badly decomposed body was found inside a Lidl bag on an allotment shed in Brighton last March 1.

The cause of death was unascertained.

Professor Peter Fleming, an expert on infant health, considered the circumstances of how the baby died after hearing Marten’s account.

Giving evidence on Tuesday, the defence witness told jurors: “I have seen no evidence of hypothermia as a cause of death but the description of how the baby was being cared for would make hypothermia exceedingly unlikely.”

He said the length of Victoria’s feet were “most compatible” with death occurring at the age of two to three weeks.

Cross-examining, Tom Little KC asked if his opinion would change if he learned that Marten had lied to him about Victoria’s death.

Prof Fleming said Marten’s description had been “extraordinarily consistent”, saying: “My assumption is she is telling the truth unless there is evidence she is not.”

Mr Little said: “Has it crossed your mind that she might be responsible for the death of her baby?

Prof Fleming replied: “Of course I have but I’ve seen no evidence of that.”

The prosecutor pointed out that the witness had not spoken to Gordon or seen CCTV footage of how the couple were treating Victoria before they went off to stay in a tent.

Read more: Constance Marten thought police would 'blame Mark Gordon for baby death', court hears

He had not been present for all of Marten’s evidence in court, during which she had said nothing about Victoria’s hands and feet being warm, he said.

Mr Little went on: “Unlike a criminal trial you do not challenge what you are told.

Prof Fleming asserted that it would be “inappropriate” for him to do so.

Mr Little said: “You relied on what she told you about the clothing she (Victoria) was wearing.

“If she had lied about those matters that would have an impact on your opinion?”

Prof Fleming said that a baby feeding well was not consistent with hypothermia.

Mr Little said: “You haven’t taken into account the fact previously she has lied to healthcare professionals.”

Prof Fleming said that was “irrelevant in this setting”, adding: “One has to assume unless you have evidence to the contrary, they are telling the truth.”

The witness was challenged on an assertion in his report that the defendants bought warm clothing for themselves and Victoria before leaving London for the South Coast.

Mr Little said: “Are you aware there were no items of clothing bought for Victoria before they left?”

The witness replied: “No, I was not aware of that.”

He was asked if having a baby sleep face down on an exhausted mother under her coat could pose a risk.

Prof Fleming said: “I would say it is not desirable but I have no evidence it is dangerous.

Mr Little asked: “In general terms, do you agree a baby sleeping in a two-person tent requires some thought as to how to seek to ensure the baby is kept as safe as possible?

“With two adults, some possessions, including pillows, a duvet and quilt, you have to be careful?”

The witness agreed.

He was asked how he would have reacted if Marten had told him they are about to sleep in a tent, with only a babygrow and vest in temperatures of between 4C and 10C.

The witness told jurors: “It would be inadvisable.

“The advice would be to keep the baby in your coat.

“I would be concerned if there were no appropriate baby clothing.

“It depends on the circumstances.”

Mr Little suggested the risk of death was “obvious”.

Prof Fleming responded: “On that scenario, if the baby is inside the mother’s coat, the baby would be protected from the thermal environment.”

Mr Little asserted: “This was not a tent you would go camping in significantly cold weather in.”

Prof Fleming replied: “It would not be ideal, certainly.”

Jurors have heard how the couple went on the run from authorities in a bid to keep their baby after their four other children were taken into care.

They abandoned their car after it burst into flames near Bolton, Greater Manchester last January 5 and were finally arrested in Brighton last February 27, days before Victoria’s body was found.

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.

The Old Bailey trial was adjourned until April 8.