Martha Daniels, aged 23 months, died last September after being left alone for three hours in a car.

Coroner Oliver Frankl recorded an open verdict after hearing doctors could find no clear cause of death.

Strapped into seat

Her mother Sarah Daniels, 24, told how she left her daughter strapped in a car seat in the vehicle outside a house while she completed some cleaning work.

She said it was not a particularly warm day but she wound down the driver's window to allow air into the car.

Mrs Daniels, from Litcham, Norfolk, said: "I was checking on her. I don't know how often. Every 20 minutes I would think. Most of the time I looked out of the windows."

She said at about 11.30am she returned to the car after finishing work.

As Mrs Daniels was too distressed to go on, Mr Frankl read out a police statement she had made shortly after Martha's death.

'Sweaty and damp'

It said: "I could see that Martha had white froth coming from her mouth.

"I opened the car door and unstrapped her from the car seat but she seemed lifeless. I called her name constantly but there was no response.

"Her eyes were closed, her skin felt sweaty and her hair was damp from the sweat."

Mrs Daniels drove her daughter to a surgery where she was pronounced dead.

Pathologist Dr Virginia Sams told the court she had conducted tests on Martha's body four days later.

Body temperature

She had been told Martha had been left in an unshaded spot and her body was limp and extremely hot when found.

But she said there was no evidence as to what Martha's temperature had been when she arrived at the doctor's surgery or what the temperature was inside the car.

She had been forced to conclude that the cause of death was not ascertainable.

Dr Sams told the court how studies showed the body temperature rose by one degree per hour if it was not allowed to lose heat in the normal way.

She said child seats could have the effect of making children hot as they prevented air circulation.

Sargent Steve Hopkinson told the court checks had revealed that the temperature that day had varied between 50 and 70 degrees Farenheit.

It had proved impossible to set up a reliable experiment to show what the temperature had been inside the car.

'No evidence'

Mr Frankl said: "Martha Daniels was a perfectly healthy little girl.

"Her mother cannot remember whether or not she physically went out to check on Martha. I suspect she did not.

"When she found her, her eyes were closed, skin sweaty, hair damp ... in those terms what happened is entirely clear. The difficulty is finding out exactly what caused Martha's death.

"Despite all the tests carried out by Dr Sams there is no evidence which could lead her to a conclusion as to the exact cause of death. I have only one option and that is to return an open verdict."