A grandmother described how she watched a mother leave her children in a car for more than 40 minutes on the hottest day of the year.

Police were called to Waterfields Shopping Centre in Watford, at around 12.40pm on Monday after receiving reports of three children shut in a white Nissan Qashqai parked in a disabled bay outside the shop’s entrance.

Hertfordshire County Council spokesman Andrew Dawson said the authority's social services department is aware of the incident.

The woman, who asked not to be named, said she and her sister were sitting on the bench outside when the car pulled into a disabled parking bay.

She saw the woman get out of the car and speak to somebody in the back but did not realise at the time that it was a child.

It was only when a disabled driver complained to her about not being able to park that she realised there were three children in the car.

She said: “I just thought she was quickly running into the store to return some stuff.

“You should not leave children in a car. Anyone could have crashed into them or stolen the car.”

When a security guard walked around the corner they reported it to him and he contacted the police.

The woman said the driver was in Next for at least 45 minutes, while the security guard checked all the other stores in Waterfields asking for the woman to come back.

Staff at the Next branch in the retail park appealed over loudspeakers in the car park for the woman to return to her car.

The woman apparently did not hear or heed them and carried on shopping for more than half an hour, it has been claimed.

When the police arrived they found the car was unlocked. Police opened the door and the driver returned to her car after shopping for the entire time.

Hertfordshire Constabulary confirmed that the children were aged between seven and one. A spokesman said officers gave the woman “strong words of advice”.

The incident happened on Monday, the hottest day of the year so far, with temperatures reaching around 33.4 degrees.

Within an hour a car can reach nearly twice as hot as the temperature outside.

Hayley Pierre, from the NSPCC, said: “When left alone in a vehicle, young children can very quickly start to get anxious and distressed and there are clear added dangers when temperatures are extremely hot as has been the case this week.

“We always advise parents and carers never to leave babies, toddlers and very young children alone – whether in a vehicle or at home.”