The mum-of-three was on her way to collect little Mackenzie from the nursery at Kippax Ash Tree Tree School at about 3.30pm last Tuesday, when she saw him walking alone on a public footpath.

She told the Express: “I actually had to look twice, I didn’t think I had seen it right. I was in a state of shock.”

The 30-year-old, of Greenfield View, immediately bundled three-year-old Mackenzie into her other son Harry’s push chair and went to the nursery, where they had started to search for him.

She said: “Obviously I was in a massive state of shock and was quite loud. I was hysterical and couldn’t stop crying. I have sent a letter in asking for an official investigation to be carried out to find out what happened.”

The school has stressed that Mackenzie was off the premises for less than two minutes and did not access a road but was on a public footpath which ran between the school playing fields and the school buildings.

But Clare says that is still no excuse and has taken Mackenzie and Harry, four, out of the nursery.

She said: “He still shouldn’t have been on a public footpath. He was in their care. As it stands at the moment, I have no plans to send Mackenzie to another nursery. Not yet, it’s too soon.

“Every day since, when I look at him I feel nauseous, thinking about what could have happened to him. I get close to tears every time I think about it.

“Everything has been going through my mind – what if I had been seconds later, what if I’d come in my car that day. It just doesn’t bear thinking about. My heart was in my mouth.”

A spokesman for Kippax Ash Tree Primary School said: “The safety of our children is of paramount importance and we are conducting a thorough investigation to discover how this happened.

“We have apologised to the child’s parents and can assure everyone that we are urgently reviewing our systems with Education Leeds’ health and safety team to ensure it does not happen again.”